How many times have you replaced the TopCase on your unit? At some point you should try and see if you can push for a (1) replacement or (2) refund.
It has only had 1 previous top case replacement for a stuck spacebar. I did not notice the popping issue with the original keyboard, but it had other keyboard issues (non-responsive keys, multiple hits w/ single press, stuck spacebar). I actually do not want a top case replacement because I don't think it will fix this issue -- trying to get them to just shim the problematic keys instead.
I asked if they could just sell me the shim kits so I could fix the keys myself. Of course they said no to that.
The machine is already 8 months old so I don't quite think it's fair to ask for a refund. The replacement is likely to still have the same problem, so.... I'm kind of stuck at this point. I just want the keys fixed. If they can show me what the shim actually looks like I can 3D print the damn things and glue them on myself. I really don't care. I just want this fixed.
I have a 2017 rMB that has no KB issues so far, so I am thinking this is mostly a QA issue and the design is actually OK (+/- engineering tolerances).