Well, something similar happens for 2 things:
- You mass place X blocks but you don't have that many, a portion of them will get removed afterwards.
- Obtaining slabs as a standalone block (by using admin commands), placing them will auto remove them almost immediately.
In both cases it's the server that's checking what the client does, and disagrees with it. The first time the server says you can't place that many blocks since you don't have enough and it removes them, the second time the server doesn't allow you to place standalone slabs so it also removes them.
What you 2 experience sounds like the same thing to me. Something causes the server to not like you and removes any block on placement ^^
When you have this, does it happen for every block you have or only for a specific one?