Is there a way to set the red flag in mail after you have read the mail

I use the red flag extensively as a means to identify email messages I must get back to after a quick search through mail to see which messages are important. My iPhone allows and keeps red flags when I say mark...All other versions of the iOS on my iMac have done the same. After the update to Big Sur, I can "flag" the individual message but it does not put a flag on the message! It seems they go into a separate mail box but it does NOT get flagged. Is there a way to flag these individual messages so that when I clean out my unimportant messages I will recognize the important ones? Thank you.