If an allay flies into a block you have just placed, it will make the block disappear. I have only seen this in creative, however. The bug can be recreated.
HOW TO RECREATE:
1. Spawn an allay and give it an item.
2. Now, when the allay is flying next to you, you can start placing blocks, and eventually if one flies into a block that you just placed a split second ago, the block will disappear permanently.