I use outlook 365 on my laptop and To Do on my iPhone and iPad. For many tasks, I have removed or changed due dates only to find the original dates reappear the next day. Everything will be fine for a period of time after I make the changes from any one of the 3 devices but the next morning the original dates will be there. The tasks are generally not reoccurring ones. I have tried uninstalling and reinstalling To Do on my mobile devices but no change.