An action with 'Discovery' as event source and only one operation of of type 'Add to host groups' seems to implicitly add missing hosts and to update existing hosts.
I would expect it to add existing hosts to the host group only - without adding missing hosts or updating proxy settings of existing hosts.
In my case all hosts are monitored by Zabbix proxies.
Hosts are added to Zabbix via auto-registration which makes sure to configure the right proxy.
Since I have a dedicated proxy per data center of a network and due to the limitations of auto-registration actions in terms of conditions, I wanted network discovery to do fill the gap.
Now when an existing host gets discovered by mentioned kind of network discovery rule it seems to do more than just adding it to the desired host group.
The result is having a gap for item values (~4min, possibly caused by configuration sync interval) and having auto-registration actions executed again.
For discovery actions I'd expect hosts to be added or its proxy configuration to be updated only when 'Add host' operations are used.
When I'm right with my conclusion but this behavior is intended, then this could possibly pointed out more clearly in the documentation.