I've only noticed this with my cipher, mages and clerics seem to be OK. Quite a few times my Cipher will go through the whole motions of casting a spell, voice, hand waving, spell glow, only to then go back to reloading his crossbow. No spell is cast, and no focus is lost.

 

Is this a bug or something that can just happen with this particular class?

perhaps he was interrupted? in which case he fails to cast but doesn't lose his spell or focus

I've noticed it happening, but normally it means the target they were casting at is unavailable now, or they were interrupted.

I've seen it happen too, and I don't think there was an interruption at the time, and the target's still alive.  It seems to happen more with the longer cast time spells.