Let’s assume it is not that, even though the Win10 October Update tends to delete/corrupt a ton of User Files due to a bug with Intel Drivers.
This sounds like it only renders correctly when an Entity is onscreen, and last time I replicated the Issue I was in Multiplayer on a Server and made sure that no Entities were closeby (only got Squids at times). This might be the hint I need to replicate it at the very least.