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I had two end-grain cutting boards that cupped about 1/16 inch after gluing. So one side is concave and the other side is convex.

If I let it sit, it stays the same.

If I plane it flat, then it cups again to 1/16 inch and stays there. But now is thinner.

I tried putting a wet towel on the 'in' side and then it flattens itself out in one day. But then it went back to cupped 1/16 inch again two days later. Any way to fix this?

(Editing to add images)

This one I tried to wet, and it went flat, but then it went back to cupped Pecan full picture

Here is the cupping in one direction Pecan cupping 1

And here is going the other way. The other side is opposite, so it's cupping out instead of cupping in Pecan cupping 2

Here is another board that is cupping. This one is already oiled so I can't cut it and glue it again but you can see it as another example Ash walnut

Here it is cupping long ways Cupping 1

Here is cupping short ways. Same as first board, the opposite side is cupped out instead of cupped in Cupping 2

UPDATE:

After two weeks the cupping did continue to get worst. The cup grew in size to nearly 1/8" and then stopped growing. I have waited for 2 months and the board has not cupped any more since then. Now I will flatten the board once again. I will report back in 1 to 2 weeks after this new flattening.

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    You might be able to cut it up and glue it back together to minimize the cupping stress on the board...not sure you'd want to do that, but it might work. – Greg Nickoloff Dec 23 '19 at 19:21
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    Hmm.... unpalatable...cutting board....might be a pun in there somewhere.... – Greg Nickoloff Dec 25 '19 at 16:43
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    The idea is, that by cutting it you can isolate the cupping within narrower strips. A 12" surface will.show more noticeable cupping than six 2" surfaces. I don't know what to think about the stress "is in the joints" but I don't necessarily agree with that assessment. BTW, it might help if you posted pictures of the cutting board so that we can see what you have to work with. – Greg Nickoloff Dec 27 '19 at 13:52
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    "The blocks are in a random pattern" this is an important detail that should have been included in the Q. You can edit it in now so that potential respondents have more info. Why not just post a photo or two of the boards so we can see what's going on? – Graphus Dec 28 '19 at 8:56
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    Want to add, if these boards were made for home use I'd just use them as they are if they won't flatten — I use a handful of boards in rotation when I'm cooking a lot and a couple of them don't always return to flat after washing. If the board doesn't rock I'd just use it crown side up, if the warp is uneven so the board rocks corner to corner I'd either install rubber feet or rest the board on a folded cloth/towel (this is a common cooks' tip for boards that don't have feet, so something a user might do anyway). Obviously the boards become one sided which is unfortunate but can't be helped. – Graphus Dec 29 '19 at 8:23
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It can depend on WHY it's cupping. if there is something in the wood, such as stresses that are being released, not much you can do. It doesn't sound like it's a poor job gluing it up, (as in to much pressure on one edge so the glue job leaves it with an arc). Especially since you planed it down after gluing it up.

Now there is one possibility which might be fixable, and that is moisture. In this situation, it's possible that the wood wasn't completely dry, or the glue was very wet, as the board dries (unevenly) it will cup up from the edges, since there is more exposed surface area that way. Now if it is a moisture issue, ideally, drying the whole board down to the same moisture content 'should' leave it flat again.

using an oven can speed this up, but just turning the board over and leaving it 'dry' upside down should also let the other side to catch up on it's drying. This is only a 'chance' but it is a fairly simple test that you can try before taking more drastic actions. It also means that if the cutting board absorbs and releases moisture through the year, it will continue to flex and relax...

ETA

when the board is laying on one side that side dries out slower. kind of like a half an apple with the skin still on. as the one side dries faster because of more exposure to air, it shrinks faster pulling the edges in and making it cup.

enter image description here

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    @Xuan, "If the problem is moisture not being even, then why does it go back to cupped so fast? " The structure of wood is likened to a bundle of straws, so end grain is like the open end of the straws. Some species are more open than others (red oak is a particular example) but all woods take up and lose moisture from the end grain with surprisingly speed, much much faster than from long-grain surfaces. An end-grain board of course has a huge amount of this fast-drying surface exposed which is why they can respond so quickly to moisture changes. – Graphus Dec 29 '19 at 8:29
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    @Xuan, as Graphus points out. because of the end grain and your other comments, I am thinking it is more about uneven drying of the wood. Stand them on edge like Graphus suggests and let them dry evenly on both sides, I suspect they won't cup anymore. – bowlturner♦ Dec 30 '19 at 17:29
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    @Xuan when it is laying down on a flat surface, the 'top' absorbs and releases moisture faster than the bottom that is 'protected' from evaporation. if both the top and the bottom are equally exposed to the air (say standing on it's side), you will see little or no cupping because both sides should dry equally and pull at nearly the same rate. So getting the whole board to the same lower moisture content is the goal. – bowlturner♦ Dec 31 '19 at 14:40
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    In addition to what Bowlturner says above, there's always a moisture content (MC) gradient within wood between various surfaces, with the exposed areas being drier. With normal boards (planks of wood) there's generally a gradient from surface to interior, with the inside having a somewhat higher MC than the skin of the boards. This is almost always the case and is perfectly normal. Even with the absorbency of end grain boards of this type the wood can't be expected to equalise fully just sitting flat on a surface which prevents any moisture leaving that face. [contd] – Graphus Jan 1 '20 at 8:07
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    Fundamentally this comes back to the material you've used which was my suspicion I refer to above. If your workshop area is noticeably less dry than your home and this is where you store all your wood, the wood in the boards is trying to equalise once it gets in the house and can't do it equally. Once the wood has had a chance to equalise (reach equilibrium moisture content or EMC) the boards should be safer to resurface and then use without further warping. If this is the case just to mention it may take longer than a week for the boards to stabilise. – Graphus Jan 1 '20 at 8:11

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