I think the short answer here is: Bark is never permanent, unless it's alive. One the one hand, imagine bark as your skin. The surface of your skin today is not made up of the same cells today as it was a month ago. Bark is very similar in this way. While it's living, it is constantly replenished and growing. Once this 'living' process is halted, the bark will break down and decay.
Certainly, there are all kinds of polyurethane, shellac and oils that will preserve the life of the bark on the walking stick; but, unfortunately the wood and the bark will decay, dry and change dimensions at different rates. Eventually the bark will mechanically separate from the wood.
I think it might make more sense in this case to either strip the bark to finish the wood, or leave it natural. Eventually it will separate, and that is ok.