It’s not a bug, it’s a feature.

Readability (or Safari Reader) give points to wrapping elements depending on the content and then takes the wrapping element with the best score. A problem it would be having on this forum is that there is no real wrapper element that contains all content. (It ignores the BODY element because for most sites there is just too much in it.)

For the HTML of this forum it means every single reply is fighting to be the content displayed by Readability.

And yes, the idea with this topic was to get to some kind of baseline for displaying running text. Most readers will not know anything about setting their own style