Oh, man, how did I miss those? Thank you! I must have looked at those options a dozen times while investigating and not realised that they weren’t all checked before. Unchecking the top one fixed it, obviously. It’s right there in the words!
I should have thought to double check every setting because one or more recent firmware and/or app updates managed to change a whole bunch of random settings e.g. Nova Launcher was showing app tray tabs in the wrong order, and one of my calendars had its sync option disabled.
A bit OT, but is this the sort of thing that happens occasionally with Android? That firmware and/or app updates can alter settings? I must confess Android still feels new and confusing to me, despite using it for months now. I suspect some aspects of it always will.