Reminders no longer move to the top when you uncheck them?
I had the same issue going to iOS 14, but there's actually a good reason: Reminders now has a few different sorting options to choose from, and the default seems to be ‘manual’ which won't do what you (or I) want.
To always keep unchecked items above completed items:

Open the list of items
Choose the ellipsis menu from the top-right
Tap Sort By
Choose Priority
Choose the ellipsis menu again
Tap Sort By
Choose Highest First