I have Office 365 Home Edition, so I've got Outlook 2016/2019/365 (don't know how it should be referred, so one of those will work. :) ) I've been using Office for over 10 years. And I have depended upon creating tasks for all of that time.
Recently Outlook tasks have been acting very unreliably. For example, several times I've created a 1 time task - i.e.: no recurrence whatsoever. I've saved it and then when the time comes for the task to come up to remind it, I have always, until now, right-mouse button clicked on it to select "Mark Complete". It is the marking it complete which is not working, very often. Instead, what happens, it the dialog box with the task will disappear and then immediately pop up again. I can dismiss it over and over and over and over and over and over and over and over again ad nauseum, to no effect. That task and most other tasks which are of the same type (one shot, no recurrence) are behaving like this. The only way I can stop it is to dismiss (i.e.: delete) the task. I don't want to do this, because then I lose the history of having made a task. I've even gone so far as to open these one-shot tasks, just to make sure that somehow a recurrence wasn't accidentally created. They haven't been. So, I don't understand why it is never allowing itself to be marked completed?