facing issue while casting

[Ri_ri]

That's the game mechanic. It's like that for monsters too (running behind the Bavarois for example)

Wait until they stop to start casting.

I don't know if you two are aware but you need to hit the "face" button to continue a spell if someone runs behind you or out of your LoS when casting a spell, however i do think it is fairly easy to just hit said button if you notice you're not facing the target any longer.

We have one?
I don't play with a mouse, so I had no idea.