Problem rasterising unicode-named spot colors
If you make a spot color named not in English, assign it to a path and then try to rasterize it with "keep spot colors" option checked, the rasterized image will miss that spot color. It will be converted to process colors instead.
Renaming it to English name immediately fixes the problem. Adding a single non-English letter (I tried Russian) returns it back.
This can be easily tested with separation preview panel that hides unused spot colors. Tested on Mac with the latest Illustrator, also tried on older Illustrator CC - behaviour is the same.