Tested on 4.12.2. It persists either until I load an older save or if I start a new map. And by that I mean "changemap" does not fix the problem. It seems to be completely random as far as each individual map is concerned and I can't tell why it's happening. PSprites are rendered, dynamic lights seem to affect them, but literally nothing else gets rendered. It looks deterministic and I can reproduce it 100% of the time using the buggy save game. If I don't use i_timescale 15, it taker longer for it to occur, meaning it's probably something tied to the gamesim. I'm unable to provide the save because it has far too many mods loaded in it, but I'm familiar with all of them so I highly doubt it's those. Not to mention HOW could a mod even cause this in the first place? Plus until 4.12 I had not seen this problem occur. If there is some additional debug option I can turn on, I might be able to provide more information.
This is a screenshot from just before it occurs.
Screenshot after it does. I have not moved a single pixel between screenshots.
Only noteworthy thing I can see is that Walls goes to 0, but for some reason the number of rendered Flats is increased. I can even pull up the automap which renders just fine.