Just curious, did you use these with cards which doesn't have 6 or 8 pin aux pcie power going into them?
Actually, I don't think this really matters - consider all the news about the RX480 consuming more than 75W of power off the PCI bus side.
Consider what the spec has as limits for the different connectors:
## The floppy connector that they use to connect to the riser is max 36W
## The those SATA connectors they give a table for are max 54W
## The more traditional 4-pin Molex is 60W
## The miner-staple 6-pin PCIe is rated at 75W
## The GPU specific 8-pin PCIe is rated at 150W
So that's why you see the problem with something like the RX480 - it's already pulling more than 20% over what a Molex-based riser is spec'd to supply.
Keep in mind that it's certainly possible to pull more power over many of these connections, but you're limited by your weakest link. If your Molex cable has 22AWG wire, then the wire is probably going to fail before the connector - but if you're running 16AWG (miner grade) PCIe cables and are pulling 150W+ off of a 6-pin PCIe, then it's probably going to be the connector housing that fails (which is what you see in your example).
So the moral of this story is be aware of whatever you're plugging in, and do some monitoring of power consumption of each new type of card so you know what it's general characteristics are, and whether it should be something you keep an eye on or not.
