Verified with Satellite 6.3 SNAP 10: satellite-6.3.0-16.0.beta.el7sat.noarch, 
tfm-rubygem-katello-3.4.4-1.el7sat.noarch

hammer> host subscription register --name dummy.example.com --lifecycle-environment-id 1 --organization-id 1 --content-view-id 1
Host successfully registered

2017-08-09 12:50:31 243411fc [app] [I] Started POST "/api/hosts/subscriptions" for 192.168.121.35 at 2017-08-09 12:50:31 +0000
2017-08-09 12:50:31 243411fc [app] [I] Processing by Katello::Api::V2::HostSubscriptionsController#create as JSON
2017-08-09 12:50:31 243411fc [app] [I]   Parameters: {"name"=>"dummy.example.com", "facts"=>"[FILTERED]", "lifecycle_environment_id"=>1, "content_view_id"=>1, "api_version"=>"v2", "host_subscription"=>{"name"=>"dummy.example.com", "facts"=>"[FILTERED]", "lifecycle_environment_id"=>1, "content_view_id"=>1}}
2017-08-09 12:50:31 243411fc [app] [I] Current user: foreman_admin (administrator)
2017-08-09 12:50:31 243411fc [app] [I] Current user: foreman_admin (administrator)
2017-08-09 12:50:31 243411fc [app] [I] Current user: admin (administrator)
2017-08-09 12:50:31 243411fc [app] [I] Authorized user admin(Admin User)
2017-08-09 12:50:31 243411fc [app] [I] Current user: admin (administrator)
2017-08-09 12:50:31 243411fc [app] [I] Current user: foreman_admin (administrator)
2017-08-09 12:50:31 243411fc [app] [I] Current user: admin (administrator)
2017-08-09 12:50:31 243411fc [app] [I] Current user: foreman_admin (administrator)
2017-08-09 12:50:31 243411fc [app] [I] Current user: admin (administrator)
2017-08-09 12:50:33 243411fc [audit] [I] [dummy.example.com] deleted 0 (5.4ms)
2017-08-09 12:50:33 243411fc [audit] [I] [dummy.example.com] updated 1 (24.0ms)
2017-08-09 12:50:33 243411fc [audit] [I] [dummy.example.com] added 0 (1.2ms)
2017-08-09 12:50:33 243411fc [app] [I] Import facts for 'dummy.example.com' completed. Added: 0, Updated: 1, Deleted 0 facts
2017-08-09 12:50:33 243411fc [app] [I]   Rendered /opt/theforeman/tfm/root/usr/share/gems/gems/katello-3.4.4/app/views/katello/api/v2/hosts/show.json.rabl within katello/api/v2/layouts/resource (11.4ms)
2017-08-09 12:50:33 243411fc [app] [I] Completed 200 OK in 2349ms (Views: 18.3ms | ActiveRecord: 41.1ms)

Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA.

For information on the advisory, and where to find the updated files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2018:0336