This block in web.config includes the "phpbb" segment:

Problem is that this apparently doesn't just apply to the root directory. It carries forward to sub-directories as well which is a problem for official phpBB extensions since the vendor folder is "phpbb". The end result is that client requests for .js, .css, and any image files under that directory come back with a 404 response from the server.

An obvious/temporary fix is to remove that segment, but that would eliminate the whole reason of adding it in the first place. Not yet sure what the proper solution here is.