TB 78.6.1 on macOS 11.1. Whenever I view a message for which Thunderbird shows a Sender header in preview or message view, from that moment it shows that exact Sender header on all message views/previews. Right after the start of Thunderbird, it's O.K, i.e. it doesn't show a Sender header. Then I view a message from a mailing list for which Thunderbird thinks it has to show a Sender header (the -bounces address) from the Sender header in the mail. O.K. for that specific address it's correct. Now if I view any other email in the same folder, other folder, or other mail account it always shows that exact Sender header from that mailing list. It always shows that header, regardless of whether the shown message does have a sender header or not. Until I view another mailing list email for which Thunderbind thinks it has to show the Sender header. Now the header shown changes to the new Sender header of that mail and from then on it always shows that Sender header. I have only noticed this on some mailing list emails, not on all. It doesn't show the Sender header (or changes the shown header) on all mailing list emails, but only on some, even though all those mails contain a Sender header. So basically: occasionally TB thinks it has to show the Sender header. When it does, it shows that Sender header on all emails until I view a different email for which it thinks it has to show the Sender header. At the moment, it's unclear to me when TB thinks it has to show the Sender header. It's for some mailing lists but not for all even though most mailing lists do have a Sender header. But worse: once it shows a Sender header that "sticks" in the view until it changes or you restart TB.