Graphics bug - some monsters are invisible (until engaged)

[sullen bone]

Reporting a bug when one type of my monsters (noticed so far), can sometimes/often be "invisible", until they are engaged. This impacts the normal mobs as well as the elite mobs of that type.
In this case it's the monster I've called "Necromancer".
This occurs both on the map and in dungeons; but also happens inconsistently (e.g. some of these mobs never seem to go invisible; other do).

Hard to screenshot invisible monsters, ha.
In these attached screenshots, the first is my prototype dungeon; every single "Necromancer" in it is invisible (makes it really hard to delete them!). The second is one of the instances, where every "Necromancer" is visible.
In another case, one of the elite monsters on the main-map went invisible, but I had a quest pointed at it already, and the arrow correctly identified where it was. It didn't re-appear until a toon reached it and engaged it.

Afaik the "Necromancer" model is using the default-generated "Sage" sprite, with only some color changes.
I have submitted an in-game bug report from the dungeon prototype screen which is impacted.

[keen perch]

Are they always invisible in the prototype, even if they are visible in one or more of the instances? Or randomly visible in either place?

And you say you've seen this on the overworld, is that with the same creature(s)?

[sullen bone]

From what I can tell, they are only invisible in the prototype, and they are visible in every instance that is generated. This maintains even if I "close" the dungeon and re-open it, every instance being generated with this specific creature being visible, while still being invisible in the prototype. This is true for the normal mobs and the elite mobs of that creature.

If I place a new one of this particular creature in the prototype, it maintains visibility (in the prototype) after re-activating the dungeon.

I believe I've seen it once in the overworld, with an elite of the same creature; however I haven't been able to repeat that.

I just saved and restarted the game (full exit, restart), and the creatures have returned to the prototype. So I could consider this one "solved", though unknown as to what caused it in the first place.