You stop because it cancels the attack to cast the spell. Then by default it auto attacks the closest person.

I too have noticed this but even more infuriating, sometimes I'll click a number to prep one of my skills and then click a demigod and it wont cast for some reason -- even when I've got all the mana in the world.  It might be bugging because they're out of my immediate range?  If that's the case, it should always attempt to move me within range, which usually it does.

It's very strange but it's been noted by me as well as a few other players before -- specifically with sedna's heal but not limited to that at all.

There's definitely some targeting bugs still lingering in the game that are absolutely infuriating.  They've lost me kills or gotten me/my teammates killed before.

Yes, I have had issues with foul grasp as well!  Also Sedna's pounce sometimes doesnt go, and more troubling it and other abilities like it cancel attack, so then the AI defense-attacks the nearest target.  Incredibly infuriating when your killing blow is off by ~100, you're still in range, and one more hit woulda done the trick -- but a priest or archer is closer to you so you attack that instead.

I had an issue last night with Sedna where I wasn't able to heal for the first five minutes of the match. I literaly had to move across the map and come back before the ability would work. It was quite annoying.