"Stuck" RB bumper after reassembly

Hello!

Ive recently taken apart two of these controllers. I cleaned them and did minor repairs.

After the reassembly i experienced the exact same problem with both of the controllers. The RB-bumper button got somewhat sticky and doesnt always release. It also became somewhat unresponsive when clicking the button on the very edge. The LB didnt show any of these issues.

I've tried:

Inspecting and cleaning any parts of the controller that could interfere.

Replacing the bumper itself.

Removed the trigger buttons and cables to the vibratingmotors. (issue persisted with them removed)

I already ordered a new plastic house or whatever it's called. But when my second controller got the exact same issue, I'm not so sure that will solve anything.

I have seen on some videos that there is a plastic membran between the actual trigger button on the motherboard and the plastic teeth on the bumper. I did not notice these in my controller when taking it apart, could they have been lost and cause this issue?

Any help is appreciated.