I suspect this is one bug, fairly widespread in one form or another, that you’ll just have to live with, unless of course Avast can come up with a solution. I subscribe to the Scambusters newsletter, and for whatever reason (probably just the subject line), my Thunderbird always insists on warning me it’s a suspected scam. so I have to hit “ignore warning” to continue.
Apparently this is independent of the learning-curve of the junk filter … if it would actually treat it as junk then I could simply mark it “not junk” and it would remember the sender’s address as OK.