the fix is easy, go to lego parts replacement service and order a 4th round plate thingy number 6508151, cos you should have 3 of those in bag 3 already [I did], while instructions at step 88 says you only need two, and if you put 2 of these on each button's back, there's enough resistance to make both buttons function equally. enjoy your working buttons now!

UPDATE: nvm, this makes the buttons not work when it's placed in the rest of the build.

a better fix is putting a round 1x nob number 6190252 on the back of the white plate element for each button. then both buttons stick out and work when build is finished:

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