Seems to be happening again with my cleric, although this time it's just sitting on the bugged mob indefinitely until the bot afks out or gets killed by an invasion or rift or some such.
What happens is the mob falls through the world, but when the bot gives it up as a target, it doesn't seem to move on (because its still in combat) and just sits there waiting to be attacked by whatever its in combat with.
Ultimately this is a problem with Rift itself and hopefully it'll get fixed in the near future, but until then a workaround (if possible) would be nice.