If you try to place a chalk mark on a block that is partially obscured by another partial block (slabs, stairs, doors, etc.), it will succeed and in the process delete the partial block.

I first noticed the issue with the 2.0.0+1.18.2 build, then tested with the latest code (and no other mods) and took the following screenshots:

Nice find! Thank you for reporting this. I will get that fixed.

Fixed in 2.0.1 via 12d3bf9