To repeat: Create a group of platforms, create 2 alert defs on the group that always fire. Wait for them to fire, delete the 2nd alert def. The history for the alert has disappeared (should be there). Then re-create the alert def with the same name if it's still not misbehaving.
rev5092 - change the semantics of the group alert history page to display any alert under any resource in that group (as opposed to only alerts whose corresponding definition while a child of some active group alert definition);
Alert defs that are removed from the group still retain their history. However removing resources from the group deletes that resource's alert history from the group history. I think that's still wrong, but will need to be fixed separately (according to joseph, requires big changes) rev5122
This bug was previously known as http://jira.rhq-project.org/browse/RHQ-2324