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 and entity properties disappear. 

As seen in the first photos 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. 

In the second photo, I am not in a block but behind orange glass. The writing on the left sign disappears. This had nothing to do with chunks or render. The text to the sign to the right is fine.

In the last photo, I show that if I move forward out of an enclosed structure the text appears.