@feditips
That was only happening for a couple of days around Aug 6 when I posted the complaint. It hasn't been doing that lately.
What it does still do is this: when at the top of my feed (in slow mode) I click to see the new posts, it drops me at some random point between where I was (the topmost toot visible pre-click) and the newest toot post-click (i.e. somewhere in the middle of the newly-visible toots).
I'd rather it left me at the last toot I already read, with new ones above.