Knowing that there is already a confirmed bug where you cannot get rid of those priority marks even by removing priority as a used field, I've set the default value in settings for priority to negative (-1), so that as new tasks are created, they would be assigned that priority
and the associated light grey shaded priority mark. Unfortunately, this isn't working, new tasks are being assigned a high or top priority and the associated orange priority mark. I have to manually change them in order to change the priority mark value and color. Probably part of the same bug that ToodleDo has already confirmed and is in the process of fixing, just making sure.
I was unable to reproduce the bug of the default priority not getting applied. Can you please tell us how you are adding the new task (quick add, regular add, website) and what page you are on when you add the task?