I thought feet can be open or closed, whether they are darning feet. The darning foot has a spring in it which bounces along, or two springs. I think Leah Day uses one which she cut off the closed part of. But I have not heard of FMQing with a traditional foot, the kind that looks like a pair of skis, either open in the front for zigzagging or closed.
Did you just change the foot part of the darning foot so you still have it bouncing along?