G'damn posterization

I'll post a few pics later when I get home. Just wanted to vent a little.

Goddamn posterization!

Problem 1: LR RAW -> PS 16-bit TIF -> LR

I have a set of portraits I took with a dark background. Nikon D610 RAW into Lightroom. No problems. I set the white balance, choose a Camera Profile. I need to extend the background in a couple of the shots so I choose Edit in Photoshop. Edit in Photoshop is set to ProPhoto RGB 16-bit TIF. Picture opens in Photoshop and some of the darker areas of the background show posterization and subtle color shift. Thinking it's just a weird display problem, I save the file as a 16-bit TIF and open in LR. F'ing posterization and color shift is still there.

I thought this pathway was supposed to be cleaner that this?

Problem 2: LR RAW + Vignette -> Export to JPG

Same set of portraits. Dark background. Starting with RAW file. No adjustments other than color balance and Camera Profile. I add a simple subtle vignette. Looks fine in the LR develop preview. Export to Jpeg, high quality. Result? Posterization in the dark background, very visible, with an  ugly blue color shift, also very visible (to me, anyway). I don't understand what the problem is with the Jpeg export. I took a screenshot from the LR develop preview window and saved it as a Jpeg and got no color shift, and an acceptably low amount of posterization.

G'damn it.

J