I do not know how related this is, or how relevant this is....
After last month upgrading to Fedora-30, and as a part of that, upgrading to Thunderbird 68, I sometimes experience the following: * after sending a message, the copy that is put in the Sent folder is bolded/highlighted like an unread message in the Inbox; * after sending the message, two copies are put in the Sent folder, and one is bolded/highlighted like an unread message in the Inbox. These happen both in yahoo accounts and in the comcast account. These each occur only sometimes. I have not noticed any pattern to when these occur.
Bill.