Game Mechanics: Mobs fall through the earth in Chardok

[Hitpoint]

I have been wiped in two exp groups and two raids due to sarnak mobs disappearing and bringing massive trains of 40-50ish mobs back to the group. It actually doesn't seem that rare. The only reason I didn't report it sooner was that they are so common I was sure someone else must have, but I can't find it with search.

The two groups that wiped were fighting in the slave pits when low hp mobs began to walk away (in the little ditch where the very first mob spawns). Then they disappeared and brought a giant train.

The raid trains happened past the bridge in the main tunnel, before the tunnel splits to the royals and koro. In one instance the mob we were killing just disappeared and came back with a train. In the other, it fell into the tunnel below us (east of where iksar betrayer spawns) and began summoning our raid one at a time into a pack of mobs.

It's not just low health mobs that this happens with. A monk friend of mine, who is well aware of this bug and used to working around it, told me that he has lost fresh pulls that fell through the earth before reaching his group.