I've noticed quite a few new Sneaking bugs introduced with Dawnguard.. Here are a few that I remember and have had recently.

-Dagger slash animation doesn't play all the way through if you're RS is pointing towards the bottom half of the screen (I.E. you're looking down). All it is is a quick little jab, and even that doesn't seem to play all the way through. Makes it hard to know if I even attacked or hit NPCs. Two ranks in One-Handed Attack Speed might have something to do with this.

-Upon coming out of Vampire Lord form and trying to crouch (Btw the animation is completely done and I'm human when I try to crouch) shows the sneaking crosshair but your character still stands up. Not sure if you can sneak like this, as I usually correct it instead of attempting to sneak while standing.

Vampire Related bugs-v

-Sometimes if you use Vampire Sight while in VL form while it is on your favorites list, it will be removed.

I don't remember the exact circumstance, but I also got stuck crouching but I wasn't sneaking when I came out of vampire lord form.

The only thing I've really noticed is that serana gets stuck in sneaking mode while she is following me sometimes

Yeah premanent sneak bugs have existed for awhile. It's sad that some are with the vl.

If you can't get out of it in game, reloading has always worked for me

What I noticed was that it gets removed from your favorites menu if you were not in the magic mode when using revert form. Just stay floating when you revert and it should stay in your favorites.
I haven't noticed it being removed when using it in VL form and turning back while still in the magic mode.

Also noticed a pretty significant delay between when you press RB and when you change/revert.