The cause of negative borderless come out lighter than the rest of the film are:
- insufienta amount of developer. Version that was mentioned by fellow above;
- the spiral tank with impurities;
- a film molding defects.
To check the last two assumptions Cut two pieces of film (~ 3 cm). A glimpse of a charge and the other a charge overturned. If the film are molding defective of the film one piece more transparent film will come out on top and one on the other in the bottom.
If the problem comes from the tank, lighter portions will come out on the same side.

George