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.
The new Great Equalizer is the SEND button.
In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
