It appears the problem occurs when mobs do not correctly highlight when mouse-overed (when there is a distinct red aura around mob and a hp bar appears on top of screen)
Perhaps not relevant but relevant but skills used on character : Puncture, Falconry, Smokescreen, Net and Ballista… Pre 1.0 launch I noticing it occurring with Rive as well.
Unfortunately I can offer no solutions myself but there is a workaround for those suffering the same problem : hold shift when pressing lmb to perform standing attack.
Thanks for your time 8)