Trying to add fog to this image and got this result. I'm using v.15.0e. the only other thing I did out of the ordinary is that the background image is a rotoscope in the camera, where the last image I rendered, the backgroung was a layer in the chor. Otherwise there's nothing different about this from my last image, just fog settings.

hard to know without seeing the settings

but is there an image being used for fog? & what is your background color? Check your lighting setup? render settings?

Fog can work weirdly (and probably the way it's supposta) with camera rotos - I believe the image used for fog will override the camera roto, depending on the range for fog.

i stick to volumetric effects, easier to control IMO