For better or worse, the OCI Distribution Spec states that

A 4XX response code from the registry MAY return a body in any format.

And unhelpfully, it doesn't mandate that the registry include a Content-type header with the error response.

The registry client unconditionally tries to parse error responses as JSON, returning an UnexpectedHTTPResponseError if parsing fails. It should expect non-JSON response bodies as conformant registries are permitted to send them.