Messaging the mods rectified the issue as of noon today. There was a little delay due to an authentication process and waiting for a subsequent reply.
The problem definitely stemmed from the billing system as my credit card company confirmed everything was OK on their end. They also confirmed no Public Mobile transactions were attempted on the Autopay day nor the day after when I lost service.
I think I was credited the month for the inconvenience. Otherwise I'd have jumped ship.
Thank you all for your help.
I've got the same problem. I messaged the mods, but if you don't have time to wait for a response, I was able to get it working by changing plan and making a manual payment, so that might work if you need a quick fix.