From the bug: "Note: This behavior doesn't persist after reloging. Tested on dogfood."
It sounds like this bug is related to the rich webclient. Are you getting the alarms in Tbird/Lightning or in the AJAX client?
Also.. currently to remove calendar events from Lightning, you need to mark them as Status: Cancelled. You can't just delete them the standard way you would with a local Lightning calendar.
Thunderbird & Lightning - Zimbra :: Wiki