I work at the MINI car factory and the they do the same thing when the door is open, the little button goes down then pops up again. I take it your door is closing properly which i'd thought it would be. Is the button stiffer then the others to push down, only thing i can think is that the locking mechanism might not be pulling the button down as its become stiffer so its popping it back up to save you from being locked in. Might need the mechanism greasing.