If you go to a tag page that isn't already your favourite, then make that tag a favourite tag all posts on the page get highlighted.
This doesn't happen in the normal case of viewing a tag page of an already-favourited tag.
The behaviour should be the same regardless of whether you already have the tag favourited, or you're just adding it from the tag page.
- Go to the tag page of a tag you don't have favourited.
- Add that tag to your favourites (I did this by hovering on the tag then clicking the star).
- Observe all tags being highlighted as you do this. (Incorrect behaviour).
- Refresh the page to observe correct highlighting.
- Unfavourite the tag.