When powering a piston with redstone, the piston the redstone is pointing into should be powered, not the one below it. See attached gif.
The bug doesn't happen for pistons south of the redstone.
I have discovered that this bug happens differently depending on the axis, and which side of 0,0 the pistons are on. I have made a video showcasing this: https://youtu.be/TE1hoDIewVo
I also want to make it clear that I believe the top piston should fire, not the piston below, lifting the top one up.
This was performed in a redstone superflat world, originally created in 20w15a, and moved over directly to 20w18a, however, none of this redstone was created before 20w18a.