Quite correct, I’d initially assumed it was happening to custom avatars only, now it seems to be inconsistent. Updated issue title and detail accordingly, apologies.
Here’s an example of a user avatar failing:
https://www.mydomain.org:80/user_avatar/www.mydomain.org/username/40/615_1.png
The weird part is that some are rendering correctly, others have :80 added …
It’s seems a little bit like this resolved issue, but I do not have a port number in my DISCOURSE_HOSTNAME setting: