Bug or just an idiotic experiment which never happens in "real life"?
My setup: Placed CMYK image; colours = C0M100Y100K0, C0M0Y0K80, one spot colour
Step 1: Fill image with pre-defined 100% K from swatch. Colour of image changes to whatever Publisher calculates. Revert the action.
Step 2: Fill image with 80% K. Colour is completely different from the result of 100% K. Revert the action.
Step 3: Fill image right after another with the red, the spot colour and 80% K. Colour is different from step 2. What is Publisher doing here? Adding colours? I would expect, that once you change the fill, it is like setting the fill to None and then applying the other colour.