I have just noticed the same thing with my debian10 test client. My client is bound to the Library/Default Organization View. subscription-manager repos on the client shows all rpm repos but no deb repos. The server provides the deb repos on the /pulp/deb/ORG/Library/custom/… URI so it’s there. debian 10 buster repos are set to enabled in the repository set of the client. So they should be there.
I am using simple content access. Maybe that’s the reason?