I was trying to move one of tas from "working on" to "done" section while in mobile version of site and I could't do it whether "done" section was empty. It was the only scenario it didnt work as expected. I could easily move from "done" to "working on", form "working on" to "done" etc.
Thanks for your feedback. The field should come up if you try to move the card away and then back to the "done" column again. And once there is a card there the problem should go away.