I was working through the same issue with discourse fronted by haproxy doing ssl termination.
I checked Force Https, and still had the mixed content warnings (e.g. on favicon).
But when I uploaded a logo, all the warnings went away - including for favicon which I had not changed.
So maybe it re-calculates the URLs for every image when you upload any image (at least for the based system images…)?
「いいね！」 1