Also easily reproducible in vanilla, but much more of a problem in CC for obvious reasons. https://bugs.mojang.com/browse/MC-69876

Upward pushing interestingly seems to work just fine:

dang github is taking its sweet ass-time realising that this will be fixed by a pull request...

This is now fixed in the latest snapshot: https://bugs.mojang.com/browse/MC-69876

Behaviour is weirder in 1.16.2 - pistons don't seem to extend at all in mst directions between 0 and -4.

Closing due to dropping 1.15/16, and 1.17 vanilla is expected to deal with this