button does not work

[Flavor]

We have to figure out why it's like this. My first guess is that the rubber itself is either poorly conductive OR it is off-center. This can be tricky to test, but you can test things outside of the shell.

First, remove the rubber pad, and use some isopropyl alcohol to clean all the button pads. If there is any debris, flux from soldering, or anything on the button contacts, it can cause this behavior.

See if that helps. If not...

If you spin the AB rubber pad 180-degrees so that the A and B are swapped, does the problem "follow" the rubber? Is the problem now with the A button?

Maybe you already tested that, but I feel like this is different than having tested with a tester. Sometimes, if you just measure conductivity with a volt meter, it can give different results than how the Raspbery Pi will react.

To test, if you are using SSH, you can run
jstest /dev/input/js0
or
evtest

If you use one of these test software tools, then you can try different button pads over the B contacts. You can even use START/SELECT rubber on the B contacts.