Not that (I’ve never seen it?) but I think I found the trouble.
I was trying to change the preferences of two squabbling users to mutually Mute and Ignore each other. The Muting part works as intended, but the IGNORE sets MY preferences to ignore them (instead of each other). In hindsight that’s probably doing what it should, and I misunderstood the function of Ignore.
Problem resolved, I think 