I am having trouble mapping controls after the update on the rpi and, x905. Some on them will work while others won’t accept the new button it reverts to the old button immediately.
If by ‘immediately’ you mean right when you’re looking at it, hitting spacebar (or retropad ‘start’ if you have unified menu controls enabled), it will revert any setting back to default and retropad ‘Y’ (I think) will remove a mapping, so if you’re hitting any of those as part of the assignment process, it can do what you’re describing.