Anything else that might be going on here?
Couple more random thoughts, in the old datasheet it shows sensitivity with 100% helium as 244mV, whereas you get 530mV. Interestingly latest datasheet does not show helium at all.. Is voltage for the sensor running at 3V not 3V3 or 5V? not sure others see similar output levels? but higher voltage may cause heating of the sensor affecting reading (or damage it), is the gas flowing directly over sensor rather than via some flow control, could that potentially cause cooling of the sensor affecting reading, how about corrosion on the sensor structure restricting flow.. wired correctly? you do have the D side of the sensor marked "■" connected to ground, and the C side to positive supply..