And another find: We use mod_sed to add our Matomo tracker code to all text/html responses, right in front of the </head> end tag. Disabling it for Discourse (or bypassing the Apache2 proxy) solves things as well, despite Cloudflare Automatic HTTPS Rewrites active. To disabling either of both solves things. On all other pages the combination works fine, also on very large pages we have, larger than failing forum pages. So maybe the two filters, first mod_set on our proxy and then the embedded URL rewrites by Cloudflare cause something to break, related to document or chunk sizes or whatever.
We embed the tracker via Discourse theme edit now, and I additionally disabled the Cloudflare Automatic HTTPS Rewrites. There is no mixed content on our whole website. And if there is, good to see and fix it, instead of having Cloudflare masking it forever.