The reason for their incorrect placement is empty origin fields on the func_illusionary entities. It is some TB thing where empty fields are kept instead of being removed. Some entities even have doubly empty fields "" "". This causes Quakespasm to get confused about their position. It does not seem to be a problem in Fitzquake, for example. At any rate, it's something to watch out for.

I made some quick hotfix for this (which also fixes some item and monster placement conflicts): qump_vingal_entfix.zip
Just drop it in the maps directory alongside the .bsp and make sure QS has external_ents 1 (default). 

Needs to beta-test every new release. Nuff said. 

It is some TB thing where empty fields are kept instead of being removed. Some entities even have doubly empty fields "" "". This causes Quakespasm to get confused about their position.

This is why brush entities tend to be fucked up at random in my retrojam6 map :( 

Some clarification on my "clippy" comments. Wasn't meant as nitpicking but rather to point out a common problem to keep in mind.

Quake's style and architecture is big and chunky, so naturally part of the player's required skillset is the ability to 'mind his surroundings' at all time. So better stay clear of that support beam on the wall when there's a fiend approaching... Having said that, the physics can be quite dodgy and can easily make for awkward or frustrating situations. The player (as well as monsters and items) is basically a box, 32*32*56 units. And this is how he clips to the world.

Obviously, you don't have to go nuts with clip brushes and turn the map into a smooth pipe. But sometimes clipping geometry helps to make life easier for the player. In tight corridors or arenas, for example. Nothing is more annoying than getting stuck on some protruding or recessed wall part and get cornered by enemies. The AI, too, can benefit from clipped off obstacles sometimes. You know how dumb it is.
The same goes for the floor, and this needs to be stressed. Since the player is a box at high speed, small height variations can make for a bumpy ride, especially if it's some small depression, like for instance a lowered 64*64 grate in the floor. Imagine the player box walking over there, dropping down, then immediately having to go up the step again. This can impede movement or even stop it altogether. Again, not nice when fighting monsters. Check out the wodden stairs in Naitelveni's map for a particularly nasty example of dodgy Quake physics.

It's not only during combat, though. Essentially any place where it can inconvenience the player, so for example also on narrow walkways, staircases, jumping sections, and so on.
You shouldn't use steps higher than 16 units and if there are stairs or platforms of varying heights next to each other, like some open L-shaped staircase where the walking diagonally over the corner would be too high, sometimes a thin 'service clip' can help to avoid the player getting stopped where it looks like he should be able to get across.

Just a couple of examples; there are more of course. All to be taken with a pinch of salt. Although invisible, some good clipping adds polish to a map. 

:) 

Cheers for the info on the brushes there negke, good points on the clips as well. There was one area I noticed in naitelveni's map where clip brushes would have been nice (the broken tiles at the start), but he had lost the map file so geometry edits weren't really possible.

Clip brushes are great and I certainly learned how key they are during this project. A bumpy ride is not nice! 

Some entities even have doubly empty fields "" "". This causes Quakespasm to get confused about their position.
I'll look into this, can you give me map coordinates that show something rendered differently between Fitzquake and QS? 

The "" "" entries don't seem to be a problem; origin" "" ones on brush entities seems to cause trouble. Example in Vingal's map: (520 2300 60) - from the cave you start in, walk down the corridor and through the door, then just to the left there's a floating illusionary. You can find these in several areas. 

I don't see anything wrong at 520 2300 60 which is near the demon face:
https://i.imgur.com/gebZYms.jpg

but these torches right before the first door are missing their holders:
https://i.imgur.com/epFINpS.jpg

The holders disappear with QS 0.92.1 win32, and are visible with Fitz0.85
and QS 0.92.1 macOS so it's some kind of nasty platform-dependent thing. will post further in the QS thread when I see what exactly is happening.