First, this is a very ambitious mod and I want to thank you Stellar for being the one to make it happen. It works shockingly well for a game not built to have pretty much anything happen faster than sixty times a second.

I have done some testing and determined the bug can occur:
- With High FPS Support (HFS) as the only mod loaded
- In single- and multiplayer
- Via the magic mirror/magic conch/etc across a long distance (definitely not somewhere else on screen)
- It does not seem to trigger via pylon use, as far as I can tell
- It does not happen every teleport, but can be reliably triggered after only every couple times.

The bug has caused somewhat random spots of the map, the exact size of the player's view/window, between the start and destination to sometimes become filled in at whatever light level that area is currently at. If the Advanced World Gen (AWG) mod is also loaded, and the world is larger than a vanilla's "Large", the spots are completely blank even when there is something there but becomes filled in at max light level when visited after. I mention this interaction with AWG so as to give another angle on the problem.

I have screenshots of the relevant map areas here[drive.google.com]. "HFS Example 1" shows a fresh world after jumping from ocean to ocean a few times as well as from underworld to spawn with the shellphone. The "HFS+AWG" screenshots show the worse version of the bug on worlds larger than vanilla's "Large".

I'm willing to help with any further testing that might be needed. It's probably not said often enough to the authors of game mods, but thank you very much for the hard work it took to create this.