I don't know what to say other than 40 posts per page is a perfectly valid configuration, and I highly doubt that this problem is related to that setting. If that's really the most plausible theory at this point I am happy to change my posts-per-page setting to another value.

The fact that the authoritative database is correct doesn't surprise me. Deep down, FT does know the right answer, it's just not making it all the way to my browser. I still strongly suspect this behavior is due to a misbehaving FT server, most likely some caching scheme taking the wrong action.