Colored flags changing in Mail app on Mac Studio

In the Mail app on my Mac Studio running macOS 26, I mark some messages with a colored flag (yellow). Days later when I go to look at the yellow flagged messages, some are not there. They have been charged to a different flag color (red). I haven't done anything to the messages. How can I keep them with grew flag color of my choosing?

[Re-Titled by Moderator]

Original Title: In the Mail app, I mark some messages with a colored flag. Later, the colored flag changes without me doing any thing. How do I keep the colored flag I want on the messages