No settings switch for that "feature" found in my looking around. May be one, but I haven't found it if it's there.
I noticed that this began after the ads started being added a year or so ago. I've speculated in the past that this glitch is a "feature" of the forum hosting software. Even though I don't see the ads as a paying member, I still have the issue of the forums not always being marked as read. In the last couple of months, I have had the occasional subforum come up marked as having been read without having to refresh the page, but not often.