I found this same issue with a recently set up pass through cache with authentication. It was working with SOME image references - basically anything that wasn't one of dockerhubs 'official images'.

For example when I did an explicit pull for 'cachingregistry.example.com/debian:12-slim' it would fail with the authentication error - even though I had creds in my pass through registry and had valid authentication to the passthrough registry itself.

The solution was to add the 'library' prefix, pulling "cachingregistry.example.com/library/debian:12-slim". That resolved the authentication errors.