Screenshots after 999 overwrite earlier shots
Nevermind.
What it actually does is save screenshots after 999 into any “free” slots, and by coincidence I had deleted my first 4 screenshots, but only taken 4 new screenshots since hitting 999, so I couldn’t see yet that it was filling in the gaps in numbering.
What it really does when you hit 1000 is not let you take new ones. Which is also bad, but not as bad as I’d thought.