My situation: I had many hundreds of completed tasks in various lists. I went to iCloud to remove them thinking that would be the best and fastest way. At first things worked well, however eventually iCloud started having errors. If I told it to delete the completed tasks for a list, then it would come up with an individual error for every single completed task. Surprisingly, when it was finished then the tasks had been removed. I called Apple and they knew nothing about this.
Anyway, I mention this because I am curious if you have any experience with this kind of iCloud reminder delete actions? Also, do you think that this might be related to the fact that good task was not able to update and purge all these old completed Reminders?
as mentioned - 'Settings - Advanced - Delete Local Cache' did the trick!