I deactivated everything and found the culprit – Search & Filter Pro. It’s generally a good plugin; I’ll ask them about this.
I’ll acknowledge I was hesitant to deactivate all plugins as I wasn’t sure of the consequences, but I guess the settings are always saved for re-activation?
This raises a larger question, which I understand isn’t necessarily one for WC Vendors support, but if you or anyone else know the answer: how on earth can a plugin that has not updated for 18 days suddenly interfere like this? And how do we prevent plugins – whether S&F or any other – from doing this?