It may be possible that the diode(s) is/are being overdriven and what you are seeing is the increase in current pushing the diode beyond its peak output on the curve. I'm not sure why it wouldn't have shown up initially, but perhaps just over-driving the diode over time has resulted in damage. I would also be inclined to open the module and see there are multiple diodes in the module and if so is one is burnt out? Perhaps a driver connected to multiple diodes is resulting on the remainder being over-driven. In either case, I'd be careful pushing it over the 100% output point for long because it may result in further damage to your module.
-David
"Help, help, I'm being repressed!"