Well, it could be there’d be an issue with one of the shutter speed modes on the Mamiya.
I took my last roll of Fujicolor Pro 400H film and ran off a bunch of test shots at varying speeds and now post development, almost second nature now (LOL!), I see 3 are black while the rest appear to be OK …
I took a log of each shot so I’ll be going through said log and the negs to see if there’s a pattern once they’re dry.
Here’s one from a roll that was all black bar one shot: