Time for the traditional yuletide fixing of the CUPS setup at my parents' place.
Everything updated (even the router firmware because why not) and "power cycled". Further investigation reveals the culprit to be https://github.com/google/cloud-print-connector
Hmm… time for a coffee.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko