When we spawn a monster, we sometimes manually give it a Z-coordinate.
For whatever reason - let's say that the Z-coordinate is wrong - too high. The player is at Z-coordinate 1000, and the monster is at 5000. The server determines that the monster is at Height 5000 and says "That target is too far away."
The client incorrectly displays the monster on the ground, and the players rightfully report it as a bug.
We most often see this with Stationary monsters (Archers) and point spawns (which are manually placed and have an assigned Z-coordinate).
Note that as soon as a monster moves - the server will calculate a new Z-coordinate and properly place the monster on the ground.
These bugs are fixed as soon as they come to our attention, so please, if it happens to you, submit an appeal and select the /bug option from the menu.