On a similar note, I came across this video a while back of the 1225 from 2003. As you'll notice in the final scene pulling into Cadillac, starting about 14:40, the exhaust is out of quarter and one of the chuffs appears to be completely GONE.
I've become accustomed to 1225 having one chuff that's quieter than all the others, but I've never heard one completely disappear. Any reason for this? Is it just an extreme case of the things already mentioned?