Very strange. I'm just not getting it. Tried in Hexen. I let a minotaur run around for maybe 6-7 minutes and still all the attacks were fine. I'm using 2.0.57 too.
Edit: OK, yes, I got the effect in Doom. As you said, the attacks are still moving, just very slowly. So they are not being blocked by something physical, or a blocking line of some sort. They are just crawling along the floor for no obvious reason.
I think I found the problem: The missile spawning function spawned floor/ceiling huggers as if they could move with a vertical component to their velocity. They can't, so it could result in very slow moving projectiles when aiming at something nearby in the X/Y plane but far away on the Z axis, because most of the projectile's speed would try to go on the Z axis.