That's a bug present in sooo many places.
Dagnor's Cauldron, Iceclad... I have a feeling that the mobs who are fleeing in the water are desynched, between the client and the server. Landing melee hits or spells on them is often very hard to impossible.
Sometime the client won't let you do it because it things there's the water surface blocking LoS, sometimes it will, but then the server will veto it.
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