I will block the door to my cellar with cob (early game). I've noticed that when I break the two blocks, sometimes I only get one back. I tried to do some testing with cob specifically and it was a little hard to reproduce on purpose. I think it has something to do with if you pick up an item at the same time you break a block, then you get the block you picked up, but the block you broke doesn't drop.
I'm not sure if this issue is spread to other types of blocks as well, but I've caught it happening multiple times on accident with cob specifically and was able to manually induce it once. It seems like a bug, but I could also see it being a "drops aren't guaranteed 100%" type of thing. If it's the latter, though, I feel like there should be some mention of it in the handbook.