I don't know if it's a bug (like not being able to edit Discussion posts or comments that were originally on Forums hopefully is), or if I'm missing something. Is there no source editor for Discussions?
Discussions (and similar features) is a separate platform from the rest of the wiki. The concept of visual and source editors doesn't exist in Discussions because there is no complex formatting allowed. The only formatting you can do is whatever buttons you can see. As for migrated Forum content, I believe you are supposed to be able to edit it. However, doing so is more likely going to do more harm than good. When Forum content is migrated, Fandom copy-pasts the HTML into Discussions. However, that HTML is not normally permitted in Discussions. So if you attempt to edit a converted Forum post, it will get passed through the content filter just like a new post does and, as a result, loose all complex formatting it previously had.