I haven't tried it yet, but isn't there a separate GBC core? Maybe try it in that? I believe there's a core setting to force GBC mode as well.
Weird. I do not have Windows 11 so I don't know how that's different, but do you have your view settings to show file extensions? IIRC, Windows by default turns that off, meaning that if you rename the file and ad ".gbc" it'll instead technically be "[name of game].gbc.gb," which may cause whatever emulator to still read it as a .gb file instead.
As for myself, I tested both the demo and the rev A rom by swapping the extension to .gbc on Windows 10, and it works perfectly with color on my EZ-Flash Jr. flash cart, played on my GBA SP.