My guess... shutter problem, like second curtain slow to close at the end. Sometimes old equipment can be slow and idiosyncratic when getting started or in cold temperature. Camera may need maintenance (clean and lube).

My guess is... shutter problem, like second curtain slow to close at the end. Sometimes old equipment can be slow and idiosyncratic when getting started or in cold temperature. Your camera may need maintenance (clean, lube, adjust).