I found the following:
“The Widelux has no rectangular opening over which to stretch the film. It has no pressure plate. Rather, the film must be threaded across a cylindrical surface, and it’s held tightly against this surface by a couple of pressure rollers.”
Without a pressure plate of some sort, only film tension can keep the film in contact with the partial-circle form of the film rails.
Here’s a shot from another Widelux with the same problem.