Mail flags color
I use to flag emails on Mail.app using different colours related to the subject od them.
After upgrade to Big Sur, Mail.app semms to love red: it changes all flags into red!!!
I use it on iMac 27" late 2015.
I can take a look to emails also from my iPhone or iPad (both iOS 14.2), but I never set flags from them because I can't set color there.
iMac 27″, macOS 11.0