I dug a bit deeper and here’s what the problem is: Invert Timelines (as the help blurb states) inverts some timelines. So, with both Invert Timelines on and Invert Scroll Direction off, the main timeline scrolls as I’d expect: Space Bar scrolls from older to newer posts.
But now the Notifications and Messages timelines scroll from newer to older posts, because their display order is not inverted. That creates an IxD inconsistency. 🤔