Lightroom edit, preview, and output do not match in brightness

I have a fully colour managed work flow. Everything has been working fine for years.

I have a new set of photographs that are in studio, plain background, dark, of few colours. It's a Harley Quinn red and black traditional outfit shoot.

I have 4 shots that are very similar with only slight variations in poses.

In develop, they all are similar, face brightness values at 80%.

Press n and now 3 of them look slightly brighter .

Press g and they're a little darker. 

Export to srgb jpg. 1 looks similar though not identical to develop module. 2 look brighter, as if it was a high key shoot. The last one looks blown out and the face looks completely white.

So far, other images in the set seem OK. I've edited 10 others.

This has never happened with other sets. 

Obviously I can't post the photos in public . But there's no way to report bugs or get support.