Both still showing in stock for me.
No code required. Cheaper than last deal even without code.
Apparently 1900mah - 2000mah capacity is just a maximum rated capacity, and both will have similar capacity.
4 Pack $17.89: https://www.ebay.com.au/itm/4x-Panasonic-Eneloop-AA-Recharge…
8 Pack $31.99: https://www.ebay.com.au/itm/8x-Panasonic-Eneloop-AA-Recharge…
Here is a link to my post with my bad experience with "Made in China" duds: https://www.ozbargain.com.au/node/441634
I have decided to remove my post, as it is unfair to judge a products reputation based on a single bad experience.
NiMH batteries have a lower voltage than alkaline batteries, due to their chemistry. Alkaline batteries start at 1.6V and gradually decline, at 1.1V they are mostly flat and at 0.8V they are fully flat.
NiMH batteries start at 1.4V when fresh out of the charger, but that voltage rapidly drops to 1.25V. For most of the batteries discharge it will be between 1.25 and 1.15V. At 1.1V these batteries are flat.
A small percentage of devices can't handle the lower voltage, so they either complain of "low battery" or shut off early. It sounds like your keyboard may be such a device. Try using brand-new NiMH batteries in it, to rule out the possibility you have a bad battery.
There is another rechargeable battery chemistry that has a higher voltage: NiZN batteries. But thae batteries and chargers are uncommon, I have only seen them on Amazon US. These batteries start at 1.65V, and the voltage declines like alkaline batteries. You'll also only get a few hundred recharges out of them, but that's still many years of use.
1.65V sounds high, but it's not dangerously high. Non-rechargeable lithium AA batteries are even higher, they start at 1.7V, and I've never heard of anything being damaged by them.