I can't say much about it, except that this has happened on my realm for the second time. Additionally, I had trouble joining my realm before I noticed this.

Here are the steps I followed that led to the bug reappearing:

1. I terraformed a large area.
2. I made it completely flat.
3. Then I went traveling and explored the world for about 3 hours.
4. When I returned, I found this bug.

So far, it seems to only occur with chunks that are entirely flat on the surface. I have never experienced anything like chests or beacons being deleted along with it—I assume this cannot happen. However, this is just a guess.

Informations:

1. I was not able to reproduce this bug on a local copy of the realm world. (With Commands)
2. I play with a Ps5 on the Realm. 
3. I was only able to observe how the layer at height 64 was deleted.

The bug often occurs for me as follows:
Flatten a large area in a Realm. Make the area so that, when standing on it, you are at height 65. Leave the Realm. Rejoin the Realm. You will find deleted layers in some places. Alternatively, you can leave the chunks, go on a short journey, and then return. The same result will occur.

These are the ways I have unintentionally been able to reproduce the bug so far.

Observed:
A layer of a seemingly random chunk was deleted.

Expected:
The layer didn't disappear; it remains as I left it after terraforming.