Activation keys tab on Hostgroup does not store its value and so kt_activation_keys parameter is not available at host provision
In the activation keys tab on the hostgrouo I can select the activation key, but when I press submit and open the hostgroup again it seems like the activation key is not stored.
There is also no parameter kt_activation_keys shown at the hostgroup or a host using this hostgroup which results in the host not getting registered as content host via subscription-manager.
Setting the parameter kt_activation_keys directly at the hostgroup is not possible which is intended as far as I understood. Setting the parameter on the hosts works and lets the host register as expected.
In the production.log is no error message, just a 200 OK for the POST /hostgroups which contains the correct activation key in group_parameters_attributes.
Verified on an older test installation with Katello 3.5 that this is only a bug in 3.6.