Slow effect not ending on mobs
If you cast slow on monsters, the visual effect lasts as long as expected from the tooltip, but at the end of it the monsters do not recover the original movement speed.
They are slowed permanently.
Tested this on a player and there seems to be no issues there.
Also, could be correlated, but after casting a slow on monsters it seems that you can't mount the horse. The charge bar runs until its end but there is no effect.