Gary:

This situation you describe is a common occurrence with the IPhone charging cables also. I have had to replace the Lightning cords far more often than the older type. So far three cords in 6 months where the overheating and discoloration are occurring about 1cm past the connector into the wiring itself.

No this is not an investigation either but it is just personal experiences shared.

I just make sure that the power charger is completely unplugged at night or at least put the charger on a non-combustible surface just in case.

I may have my cables still. I will check for you.

Chris