Check out Zodiac if you want an example. Pols Voice-type enemies spawn in the Scorpio corridors and in the lower right part of Gemini.
The problem is this: if you create a Pols Voice and then try to move it into position with a script, the Pols Voice leaps to the top of the screen. No idea why. Code in question is something like:
npc spawnee = Screen->CreateNPC(whatever);
spawnee->X = something;
spawnee->Y = something else;
It's difficult to spawn or manipulate any jumping enemy in this way I think. For example: Using DD's enemy spawning script with jumping enemies they will always appear at the top of the screen.
It's the way the movement code works I think. Have you noticed that artificially damaging certain enemies results in their movement patterns forcing them off screen?
Ugh. L, should we try to fix this for 2.5? Or file it away with the other enemy movement problems for a later overhaul?
I'm not losing any sleep over this bug. It's weird, but not game-breaking. Feel free to put it on the back-burner.