could the light be leaking around the lensboard ?
and does this sort of leak happen when you use
a different lens+board?
since film is kind of expensive,
it might be fun+easy to make some quick and dirty
paper negatives to try to pinpoint what might be going on.
you can load paper in the holders and leave them in room/daylight
to see if it is the holders leaking or see if it your loading technique
(load paper in safelight to see what might be going on)
try shooting the paper using aroundish asa 6 or 10 as a starting point
and use both this fun lens with no tape around lensboard and with ..
and then with an actual speed graphic board+regular lens ...
i wish i had some other suggestions ...