I think that might be the answer to your question. The faster one is using more power so less is available to the transmitter.
As I editted:

The batteries were brand new, 16500mAh, with top current of 400mA. The TX current of RXQ2 gets at most a peak of 16mA)

The delta current between crystals is about 0,1mA. It's too small to cause an effect using this powerfull battery, don't you agree?

Thanks!