For pieces of mail that are linked (i.e. replied to), I am finding that, when I select a linked message, the text does not change if I click on another message in that thread. Clicking on any other message in the thread will only display the text from the first message selected. To have the text properly displayed, I have to click on a message outside of the thread and then click back into the thread.

This only happens with messages that have been filed. "Inbox" and "Sent" messages are behaving properly.

That's a bug in Mail.

I experience the same thing in Catalina with mail in my Trash box and, I think, my Sent box too. It may even have been perpetuated in Big Sur, but it's been I while since I reverted, and my memory is fuzzy.

Okay, thanks....good to know it's not just my software. I'll stop sweating it.