Originally Posted by Worstcase
The Cleanse part is not a bug at all, but I think the Mercury Treads part is just an unintentional side effect that will most likely be fixed.
Actually, yes, it is a bug related to a few.
If anyone is cleansed while multiple enemies are caught in the slow, they all get freed. Also with the mechanic of the spell, unless the tooltip is wrong, it goes by distance of the target starting when the ult is cast to when it ends. The slow can be cleansed, but if you are still in range, the ladder half should still stun you.