If you create a host that uses an external smart proxy as a content source, for example, and then register the host again using no content source (aka pulp primary), the content source won't actually be changed. This can cause errors during registration, like:

Validation failed: Host centos9-stream.example.com: Cannot add content view environment to content facet. The host's content source 'centos8-proxy-devel.example.com' does not sync lifecycle environment 'Library'. (HTTP error code 422: Unprocessable Entity)

My smart proxy didn't sync library, but the new activation key I was using was set to use library. This should work, so we need to make sure the content source is changed during re-registration.