This issue affects any non-full block, including trapdoors, farmland, slabs, stairs, chains, etc. Relogging or breaking the block is the only resolution until a proper fix is put out. Affects version v1.21.0.
This is still an issue as of 1.20.30, see attachment below:
Can confirm, does not work on realms as of 1.18.2.
This will happen when it retracts, too. Use a horizontal sticky piston and attach a block to it while extended. Add the falling blocks onto the block attached and retract the sticky piston. The falling block closest to the sticky piston will break.
Although I haven't tested recently, I remember specifically testing with a friend to recreate this bug. We managed to find that we could consistently recreate this by:
Player A jumps from ~10 blocks and Player B punches Player A before he hits the ground. The result is saturation health. I'm not sure, however, if this applies to GoldenHelmet's damage immunity time explanation, mainly because I recieved the saturation hearts before I hit the ground. I'll test tomorrow though, and I'll post my results.
Try to change your skin. There is a post claiming they have the same problem, but when changing your skin it lets you join.
I can confirm this also happens with sheep. Spent two stacks of wheat on one baby, never grew up. This is happening on 1.14.2
Likely the first report of MCPE-153515