Ceiling flat completely fails to render in some positions in THZ2

These images were taken seconds and fracunits apart. I have no idea why the entire ceiling disappears here, but it does.

(Note for other devs: I noticed this both in raw 2.1.15 and the current state of next, so it's not currently fixed.)

edit: removed identifying information from screenshots, hence vertical distortion at the very top

i think this is also exist when the level has been made