But some pieces are exposed. One of them started cracking.
Trying to fix this will probably only make the problem worse.
It looks like the crack is at the edge of one exposed piece that has shrunk. That's pretty typical for wood: it expands a little bit when the average humidity is high, and shrinks when it's low. If you fill the crack with more epoxy (or anything else), that same piece of wood won't have the space to expand again in the future when the humidity increases. Even though the change in the wood is small, it can exert tremendous force and might well cause a larger crack in the epoxy that would be impossible to repair.
In order to try to fix this, you'd need to seal the wood off from the environment so that it'll stop changing seasonally. You might be able to use a router with a template to remove the top surface of the exposed piece, creating a recess. Then you could mix and pour new epoxy into the recess, up to the surface of the table. You'd then need to scrape or sand the surface flush, and then try to polish the surface to get the repair to blend in. Doing that well seems like a difficult job even for someone skilled in working with epoxy, and probably not really feasible without the right knowledge and equipment.