Interesting, and I'm sorry to hear that. My JMJ lives on one of two Hercules wall hangers when it's not being played, I've had the bass 4 years now and it has been hung on the wall in all that time and there's no visible discolouration or other reaction artefacts where the hanger is in contact with the head. The wall hangers have been up for around 10 years now.
When I'm out gigging and rehearsing I use the Hercules headstock dangler type stands too with no issues. But the stands are only around 3 years old. Like you, I had an issue with a pair of older Hercules stands where the plastic clamping system was starting to get sticky and break down. But at this point those stands were something like 15 or more years old and credit where it's due to Hercules, when I contacted them to alert them to the issue they replaced both free of charge, not that I was expecting them too considering the age of them.
So I appreciate that this doesn't help in your situation and it sounds like you've just been unlucky. And for what it's worth, I'd be annoyed too in your situation.