In possibly-related news, I only just now noticed that my fork (which is the source of that PR) had an old travis-ci.org Webhook entry still configured in its settings, so I removed that.
Is there any way that could have been the cause of this?
Could a similar old Webhook be causing this, if it’s still configured on the target repository? (I don’t have access to those settings, but I can fire off an email and ask the admin to check for something, if I know where to point them.)