I currently have this issue and hopefully it can help.
My one rear seat.wont lock into place, so I took the lock/release button off. Someone else might have tried before, and when they put the button back on, the pin didn't screw into the proper threaded hole on the button, and it screwed into one of the segmented crevices instead.
The seat does latch back into place without the button on.
Now the current problem for me is to have the pin centered while I drop in the button to screw it in place. The pin flops around before I can get the button on top.
I'm going to use some strong or floss to hopefully keep the pin centered and upright enough for the button to screw on, then the thread should be small enough to pull out.