There might be some on change event on the due date widget,
Pls check that and remove the event and you are good to go.
Let me know, if you face any issues,
Hope it helps!!
If we are talking about the part below, it looks like that is not the cause. Do you have any other option in mind? Thank you for your help.
I have found the reason. In the task edit page this part remains wrong. Thank you!