Of course, this is hard for me to repro before there are unread posts in multiple threads under the same section again, but I saw this happen this morning.

I went to https://boincstats.com/en/forum/, saw a few sections with the UnreadThreads icon, went into the first such section, it had two unread threads with the UnreadPosts icon, I entered & read the first but not the second thread, did some other stuff that took me out of the forum pages, then clicked back on https://boincstats.com/en/forum/ which now did NOT show the UnreadThreads icon for that section anymore, but going into the section correctly showed me the UnreadPosts icon for that second thread with unread posts.

Yup. Just went into "BAM! Bug Report" that had the yellow icon. I saw two msgs with yellow icons. Clicked "Index" to go back without clicking on the msgs. The icon on "BAM! Bug Report" was now gone, as if I had read everything in it, but I hadn't. Going back into it still rightfully showed the two yellow icons on two threads.

I can confirm exactly the same thing as Tuna is saying. Been that way as long as I can remember visiting this site.

Weird that I just now noticed this after 2yrs... Must be that I always read all the posts in a subforum, once I was in it.