Looking at the screenshot they posted it still seems like there's almost certainly some kind of bug here, although I can't tell if it's on the Mastodon side or the Discourse side.
Even the most complex HTML should just turn into plain-text when Mastodon parses it. Not whatever broken HTML markup is going on here.
Also, when I view this thread from https://socialhub.activitypub.rocks/t/remove-html-code-from-activitypub-federated-posts/5293, why are the images missing?
Unfortunately, I can't view the topic itself in ActivityPub to determine who's at fault here, the server returns 406 Not Acceptable:
curl -H 'Accept: application/activity+json' https://red.podkasts.org/t/el-canto-de-la-tripulacion-n-36-nuevas-voces/23408/1And looking up the object in Mastodon doesn't return a result either.