Mana taken but spell not cast
 

Not sure exactly how to replicate the bug but happens quite often when I'm kiting seafuries in ocean of tears. I'll cast lightning blast and itll consume the mana but nothing happens. No resists, no damage. It acts as if it was interrupted but consumes my mana. I'll try to attach a video showing it later if explanation isnt enough

Sounds like what happens when you try to cast on something in the water.

Nothings in the water though. I dont get any messages saying unable to see target. Just finishes cast takes mana and does nothing

Hmm. I do remember AoE spells behaving quite unexpectedly if you are at a significantly different elevation than your targets. I'd take notice of when exactly it happens, noting the position of the mobs relative to you to see if it's at all related.