Steps to reproduce:

1. Create a hostgroup "Test"
2. Add a host to this "Test" hostgroup
3. Create a trigger action with as only condition "Hostgroup equals Test".
   The operation I have is to send a message to a user through a script media type, no idea if that is important
4. Fire a trigger on the host from step 2

Result:
If the trigger severity is High or Disaster, the trigger action is executed.

If the trigger severity is Average, Warning, Information or Not classified, the trigger action is not executed.

Expected:
The trigger action should always be executed, because there is no explicit condition on the trigger severity.

Extra information:
The user for which the action operation in step 3. is defined, has all severities enabled in their settings for the media type.

I have also tried adding condition "Trigger severity is greater than or equals Not classified" as an extra action condition, but that didn't change anything.

I have encountered the problem on Zabbix 6.4.6; the bug is confirmed to be still present in version 6.4.8.
Version 6.4.2 doesn't seem to be affected; other versions were not tested.