AI switching to greyscale

There may be a a bug of sorts in play here, but I wouldn't say it's a case of "illustrator mind-of-its-own."

Independently of Document Color Mode, color mode can be object-specific. In other words, it's possible to have an object with CMYK color(s) applied, one with RGB color(s) applied, and the third with Grayscale value(s) applied, all living side-by-side on an Illustrator artboard. When there is a selection active, the Color panel may display one color mode or another . . . conditionally as follows, according to my findings:

RGB/CMYK

• In Document Color Mode: CMYK, an object pasted from an RGB Color Mode document forces the color panel to RGB. Having been pasted into a CMYK document, the object's color mode has been converted to CMYK, but the Color Panel will continue to display its original RGB values until some other activity forces another Color Panel mode change. After that, selecting the pasted object displays its converted CMYK values.
• The above is also true of a CMYK-originated object pasted into an RGB document; just the other way around.
• If there is a bug, here it is: In either case above, the Color Panel will continue along in the color mode forced by the pasted object, even when another object, of the other color mode, is selected. In other words, for example, if you paste an RGB object into a CMYK document, the Color Panel will display the pasted object's RGB values, and get stuck in RGB, even if you then select a pre-existing CMYK-assigned object.

GRAYSCALE

• This is the wildcard, in that its behavior is faultless (and the way all modes should work), in either Document Color Mode as far as I can tell. Grayscale objects can live in either Document Color Mode (there is no Grayscale Document Color Mode), and the Color Panel responds as you'd want, changing to grayscale when one is selected, and back to the Document Color Mode when a non-grayscale object is subsequently selected.