If you don’t mind ( I know I am not here much anymore ) I would like to comment on the torque of the breaker. I notice that the one with the corrosion has a set screw that is not turned into is setting as much as the one beside it…would lead me to think the conductor is loose, on load arcing takes place which adds to the problem.
Now I don’t know this for fact…it is only an observation but notice the number of threads on the black conductors termination lug than on the red one. Sure looks like it was not properly torques to me…and to be honest with you that can be a problem. As was mentioned before it would be nothing different than a double lug for your review BUT with the corrosion and possible torque ( which can lead to a safety issue so don’t feel bad about ASSUMING in that case) …have it looked at as I also see discoloration around the plastic ( ps. Plastic does not corrode) so something else is going on and my guess would be a loose connection as well.
Again…I hate commenting via pictures but it would cross my mind…just a thought.