I recognized this some years ago, then it was gone somehow.
Now it's back.
The last days i spent hunting for rares in cycrow, while it happened 5 times, that invisible mobs where spawned. Mostly they spawn the "natural" way, sometimes get spawned by something like a Terror Raptor or even Grim Chaser.
They can still shoot players, attack them, kill them - but they cannot be seen. Same happened in Regant, when i took damage and no enemy was in sight (not in the wall, it was moving).
Zoning or relogging solves the problem, while most of the time players will just don't know what happens.
Also, some past-spawned enemies like the little raptors or maulers will get spawned into the textures - either the mountains or boxes or walls. Sometimes the player cannot attack them, but they can attack you.