I first experienced that in Catalina, and although I'm pretty sure it was perpetuated in Big Sur, I'm not 100% certain. It was too minor to aggravate myself over in the larger context of Big Sur. (For what it's worth, I run with SIP disabled.)
I doesn't happen in my inbox either, but it does happen in my trash and maybe my sent box.
It's most annoying!
Update: It happens to me in trash, but only with sequential messages, not when there's an extraneous one stuck in between.