BMP bitmaps are seen as RGB, lose transparency and cannot be coloured in 28.5
Place a .BMP 1 bit file in AI 28.5. It is seen as RGB and cannot be coloured.
PSD and TIF bitmaps work fine.
See the recent answers in these posts: https://community.adobe.com/t5/illustrator-discussions/assign-spot-color-to-bitmap-no-longer-works/td-p/13138271
https://community.adobe.com/t5/illustrator-discussions/bitmap-issues-with-illustrator-28-5/m-p/14619511#M407603
The bug seems to be fixed in Beta 28.7.0.65
Please try to update and verify if it now works as intended for you.
-
Josh Staples commented
I'm having an issue with the latest Illustrator using placed / embedded .EPS files that are in bitmap mode (decades worth of .AI files with placed and embedded .EPS files, pen and ink drawings mostly). The preview is very off - inverted or just corrupt-looking nonsense - and the fix of Object > Image Trace is not producing the nice results I'm used to.