@ThosGreen Looking at your research (which is very informative) there is probably more to it then just the browser…
According to Modzilla Developer Netword on <hr>:
Historically, this has been presented as a horizontal rule or line. While it may still be displayed as a horizontal rule in visual browsers, this element is now defined in semantic terms, rather than presentational terms, so if you wish to draw a horizontal line, you should do so using appropriate CSS (Note: Emphasis is mine.).
Or according the HTML5 standards at w3.org:
hr element represents a paragraph-level thematic break, e.g. a scene change in a story, or a transition to another topic within a section of a reference book.
The above means that <hr> is not strictly a horizontal line anymore, but now indicates a semantic break, e.g. a new section in a text. This is helpful for visually impaired users using non-visual readers.
This also means we should use <hr> wisely nowadays to not confuse visually impaired users.
If your use of the ‘horizontal line’ is purely aesthetic your choice to use a <div> is probably the right one.
Btw: I’m glad you “wasted” some time, because I learned something from it…