Tasks reappear after being marked as completed

[Darya_Shiganakova]

This problem started several days ago. After I mark a task as complete it reappears immediately. 
Signing out and logging in from my account did not help. Neither did deleting and reinstalling the app. 

I like the app very much and don’t want to change to any other. 
Please help me to solve this issue!

[#2]

@Darya_Shiganakova The other weird thing that happens is an email gets dragged into tasks and I watch it (the task) disappear before my eyes - up to even 5 minutes later all of a sudden the task is missing! The email is still filed where I left it, but I have to go back and drag it in again to retain the task. This is not super helpful because I dragged it there so I'd remember to do something. If it's going to randomly disappear. Does anyone know why this is happening? (It's been happening for literally ages - restarting/logging back on, etc. is NOT going to fix it.)