Delete reoccurring event only deletes the selected one.

I’m trying to delete an event. The prompt that appears is: This is a reoccurring event. It has two choices: This event only, All future events. I select: All future events. But only this event is deleted.

This event is likely to have been synced to Calendar via iOS version of Essential PIM from a Windows PC version. The event is no longer in EPIM on the PC.

Any ideas what is wrong and how to delete all of them at once?