I think it was likely caused by the cache trying to compare the tag with Docker Hub: https://docs.docker.com/docker-hub/image-library/mirror/#wha...

> "When a pull is attempted with a tag, the Registry checks the remote to ensure if it has the latest version of the requested content. Otherwise, it fetches and caches the latest content."

So if the authentication service is down, it might also affect the caching service.