Mac Mail, rules flagging mails in one color only

I'm using Mail on an up to date MacOS. To organize my incoming mails I use filter rules extensively. Some rules are supposed to flag mails with different colors depending on sender address, subject keywords etc.

This used to work flawlessly until several months ago. Since then, all those rules set the red flag only, even though *not a single rule* is supposed to set the red flag. I used red manually only.

I did not change anything. When I look at the rules, they still show the green, blue or whatever flag I put in there, but when these rules run, they set the red flag.

I did delete these rules and rebuilt them. Still red flag only. I built completely new rules to flag mails with any color. They produce red only.

This behavior started while using Big Sur. Monterey didn't fix it. I really don't want dump everything and rewrite all my filter rules from scratch, because the flagging rules make for a small percentage of all my rules. That would be a lot of work.

Any idea how to solve this would be greatly appreciated.