You could have a good look at the rubber gasket sealing the back-door, somewhere towards the top right.
If there is nothing obviously damaged, try sealing round the door with black electricians tape for half a roll and then compare the results with and without the extra light protection.
You could also combine that tape-check with blocking up the film canister window too, so you end up with a roll having four different stages of light-proofing, then see what results you get. Do all these tests in bright sunshine, like your examples above. Good luck.