As the title says, when you place a block in front of a mob's walking path (at the same time as they are walking into the spot) the block just vanishes, and the block item in my inventory is deducted. When I reload the game it corrects and I get the block back in my hot bar. But, I did a test and found the issue where the block disappears forever. I was in the nether and I placed a block in front of a walking Endermen so that the block disappears right? Then I died so that I don't reload to get the block back. So after respawning and re-entering the nether I found that empty spot with no item hovering over that spot. I reloaded then, but the block was still gone and not back in my hot bar.