That sounds like server lag. If you didn't know, every minecraft instance has to run on some sort of server, whether it's remote, local, or client. I sometimes see blocks randomly reappear after I mine them, but a few seconds late they disappear again. Your computer may have background programs using up RAM or memory while you are playing.
Still happening in 20w42a
Added my own example rather than make another. As soon as I saw lava in an iceberg I knew it was a bug.
I have killed 200 rabbits with a fire aspect/looting sword and they didn't drop a single cooked piece.
This is a very common bug. I'm pretty sure it is a rendering issue.
The first portal I found was 16 blocks under a deep ocean. There was a 16*16 chunk of netherrack blocks all the way down to the sea floor, and it was hollow. after mining 16 blocks I found the chest which was 10 blocks away from the portal itself floating at the top of a ravine. Definitely a strange world!
I don't think I have experienced an auto-save, actually. I thought that was only on Bedrock editions.