Steps to reproduce:

1. Have a discovery template which creates hosts and puts them in a hostgroup
2. Allow Zabbix to discovery hosts and add them to the hostgroup
3. Go to Configuration -> Host Groups
4. Click on the hostgroup
5. Click Update with or without making changes

Result:
See screenshot.
Cannot update groups for discovered host "4c4c4544-0039-5210-8038-b1c04f304d32".
Having a host discovered and put into a Hostgroup essentially makes the hostgroup uneditable in the Host Group config page, though since the workaround allows it, and it errors whether or not a change is made, it seems to be a bug in the Host Group interface.

Expected:
The hostgroup should allow the addition/removal of non-discovered hosts.

Workaround:
Go to the individual host configuration page, and add the host group in the In Groups section.
This works, but is quite tedious when you have many hosts to add.