I'm running Thunderbird 115.15.0 on Ubuntu 24.04. I have a recurring event that was initiating by an outside invitation. If I try to delete a single occurrence, by any means, it always reappears after a while. It disappears from the calendar and from the "Events this month" pane, but reappears after a while or immediately after starting thunderbird. "By any means" includes when the event initiator sends a cancellation notice and the cancellation is accepted, using the delete key from the "events" pane, using the delete key when clicking on the item in the calendar, or right clicking on the item in the calendar and finding one's way to "delete only this occurrence". This is making the calendar almost useless. This is a purely local thunderbird; I've never tried Caldav or whatever.

(On a 2nd matter, why do not of the "topics" for asking a question on this forum relate to my question?)