The problem manifests as follows:
1. Imagine some user group permissions (Read, or Read/Write, or whatever) shown as 'All groups' in the frontend. Basically it'a list of all the host groups internally.
2. When you add new host group, this permission set changes to be displayed exact list of groups excluding the new host group. Internally it's indeed working that way, new host group got added but permissions were never updated.

Why this is wrong:
1. This is definitely an inconsistency. When we have someone's permissions set to 'All groups', we expect all new host groups to get the same permissions when added.
2. This adds unnecessary burden of adding each new host group to each permissions group manually.

What can be done:
When a new host group is added, update all permission groups that have all existing host groups prior to addition set to the same privilege level with the new host group / the same privilege level. This will make it all consistent and easy. Maybe this behavior should be made optional.

Not proposing patch at the moment, because it requires API (hostgroup.create) change.