Steps to reproduce:

1. Go to a Host prototype > Group tab
2. Specify a group prototype, such as using a macro that is unique to that host which will be created, eg. {#DISCOVERED_HOST}
3. Wait for discovery rule to apply this change, or manually execute it
4. Now change that group prototype so that it is no longer unique per discovered host, but rather a host group per host from which the discovery rule is executed. eg. {#PARENT}

   /

   {#ZABBIX_GROUP}

Result:
See attached screenshots for the configuration and error.

Expected:
Zabbix should be able to add discovered hosts to a host group that was created by a discovery rule.

Please take note that:
If I delete the initial Group prototype, execute the discovery rule, and then add the same discovery rule back again, Zabbix successfully adds the discovered hosts to that host group.

Secondly, it should be noted that this is not the same issue as in ZBX-15012, because I am not trying to add discovered hosts to an existing & manually created host group.