If you select the option "Disable Background Images", not only background images and moving background objects are disabled, but also the background colors. So the background of crystal or L2 shadow levels is black instead of dark blue, making the terrain hard to see. Note that the minimap always displays the correct background color.
IIRC when introducing this option, we decided to still display background colors. This makes still a lot of sense to me, so I would call the current behavior a bug to be fixed.
This bug occurs both in the current stable version V10.12.15 as well as in the experimental versions V10.13.16.
Yes, this is a bug. Will look into it.
Fixed for next update.
Note that making the commit to backwards-compatible is not a mistake; this bug doesn't manifest in new-formats.