This is something I've just noticed and maybe due to 1.5 (which I had no choice installing by the way; this is the 2nd patch where tesv.exe disappears and I'm forced to update/repair through Steam, despite "don't keep updated" being selected).

Anyway, sneak attacks work fine when I'm alone and hidden; however, once I've summoned an Atronach to keep an enemy preoccupied, sneak bonuses are no longer applied, despite still being hidden. As soon as the Atronach dies, sneak damage is applied once more. I've only tested with bows and I cannot revert to 1.4 to confirm whether it's the patch. This impacts my play style quite considerably, especially when playing on Master.

Has anyone else encountered this issue or better yet, know of a solution?

Cheers.