The only places that I could find that Article Prefixes are being set to BOLD (if the article is unread), are Grid View Layout and Tile View Layout. All other layout types use the core XF .structItem-Title or .contentRow-title classes, which explicitly sets the font-weight on the label class to 400 (which is normal font weight).
Example, here you can see, AMS Index with List View layout does not BOLD the Prefixes of unread articles.
I did find the suggestion made by one of the Early Adopters to make the Prefixes Bold on unread articles, so that's probably why that behavior is still lingering on Grid View and Tile View (which are both "Grid" style layout types so the LESS templates for both are similar and don't use the .structItem classes).
Anyway, with that said, I've updated Grid View Layout LESS and Tile View Layout LESS, so that prefixes on those 2 layout types have the same display behavior as the .structItem-title class and .contentRow-title class (Core XF classes), which are used in List View Layout, Article View Layout & Carousel layout in all my addons.
I'll mark this as resolved as I did change the behavior on Grid View and Tile View and you can test again (against an unedited core xf default style) after the next update and if you find any instances of bold prefixes, you can report them individually so that they can be fixed.