Sorry to bother you all with this but like many people I'm new to the BL3 mod tools, and am learning as I go.
I've been attempting to make a basic one-window map and up until recently I've had no problems play-testing my builds.
However recently whenever I make a new build it is really broken and walls and objects keep disappearing.
Any idea what the problem is?

Here is a video to demonstrate my issue:

Thank you.

looks like a sun/sky problem

Seems like a visleaf problem. Honestly no clue how to help you besides the obvious things like redoing the map, verifying cache etc.

Okay, so I figured it out. Just had to add a umbra volume...