Persistent e-mail flags that are associated with an e-mail

I often flag e-mails. Since upgrading to Ventura, Mac Mail often often still sees the flag (i.e., it shows up in the “Flagged” favorite menu) even after the e-mail has been deleted, the trash emptied, etc. I have also tried rebuilding the mailbox to no avail. Does anyone have any suggestions on how to get rid of the flags not associated with an e-mail? Or why it happens?