It doesn't happen often, but it CAN happen and is VERY annoying. The room with the wolves and bees where half of them are hanging below the viewing window has issues. I got feared out of combat cloud. All mobs should appear on the spot where you exit the cloud. Most of them did. I killed all that I could see. But, since the room is "kill-to-pass" you have to get them all. Currently, both directions (east and south) are blocked because there must be one or more mobs outside the viewing window that I cannot see nor target. After clearing most of HUSH and non of the bosses yet, it would be extremely frustrating. But, it is really only an issue when solo. Of course, your group member could summon you if in another room. But even in a group, if all were in the same room, the only option is to get out and get a fresh dungeon. What a waste of time and effort. Please place mobs within the viewing area, despawn any summons even when feared out of combat.
P.S. Yes, I do have see invis on and no, there are no invis mobs that I can "see."