What I expected to happen was the chunks and blocks would all load as they normally would. It has happened more than once on different worlds.

What actually happened was random chunks would not load, and on the affected areas, the bedrock would be replaced by, in my experience, mushroom blocks, or regular grass.
Describe what happened here

1. You don't really trigger the bugs using steps, so I will just say what happened.
2. On one of the worlds, I had converted it to an older version, and it crashed. I thought this was because there was newer blocks from the latest updates, it had happened to me before, and it was fine once I changed back to the latest updates. But now, they just delete certain parts.
3. On the other one, I had created a regular world on 14w21b, and random chunks were not loading, and the bedrock was replaced by mushroom blocks. The weirdest thing is that i know that sometimes chunks just regularly don't load, but there is very many of these .