Thank you for your reply.
The issue is resolved.
This is good, but also frustrating, because the issue persisted and was totally consistent for months — we disabled BPS when we wanted to change the design, which was tolerable because the site wasn’t yet live — and I have no idea what happened that made the issue disappear.
If it recurs, or arises on another site, I’ll return to this thread.
In the meantime, if someone else encounters the issue, or has any insights, this is your place!
Thanks again,
Robinton