Absolutely 100% a light leak on the 645. I've had it and seen other threads with it. The light is getting in around the catch and fogging the bit of film on the spool below. Change the seals taking good care to get a good seal around the catch and all will be good.
When fitting seals, self adhesive material is great but tends to stick as soon as it touches. Trick is to wet the sticky surface with a drop of water containing a tiny drop of detergent as wetting agent. Foam will now slide into position, and sticky will only grab when it dries.
Reason it only shows occasionally is that if you wind on quickly not much light enters, if you carry the camera around in the sun you get the leak big time.