First of all, this occurs in Minecraft Java 1.16.3 but the affected versions will not let me enter 1.16.3. It says Version with id '1.16.3' does not exist.

The issue:

I found an issue that while in spectator mode blocks disappear. 

As seen in the photo I am in spectator mode in a bedrock block in the nether. For some reason one of the blocks of the game generated portal is missing.