My 3 years old 13'' MacBook Pro Retina doesn't sit flat
I had a similar experience with my 2013 MacBook Pro. The battery was bloated causing the MacBook to wobble even on perfectly flat glass.

Battery checker apps like CoconutBattery were reporting it to be normal. Even macOS wasn’t showing any battery warnings (I have consumed only a reasonable number of charge cycles).

While the cause of bloating still remains unknown to me, Apple service recommended me to replace the battery as the continuous bloating may end up causing damage to the board, bottom case, trackpad and the keyboard.

It is advisable to get the battery replaced as soon as possible.

Note: After getting the battery replacement, I sought advise on do's and don't from the technician. I was told that this wasn't an expected behaviour and happens in rare circumstances. Thus, the cause for bloating remains unknown to me.