Does the calibration sequence seem ok, or can you observe a difference (more/less noise, etc) when it is spinning to full lock in either direction?
I've had my G25 apart before and from the looks of how things are laid out in there, it does seem reasonable that one motor would provide most of the resistance for right-hand turns and the other would provide most of the resistance for left-hand turns. It sounds to me like one of your motors may have burned out or something. The two motors are always working in tandem to produce various FF effects, so you'd likely still fee some resistance, albeit weaker, even if one of the motors was dead.
It's pretty easy to take apart, and since you're out of warranty anyway, I'd recommend cracking it open to see if you can get a better look at what's going on. You might be able to observe if both motors are working properly. If you've got a volt meter, you can do some testing to narrow down the cause by determining if there is voltage making it to both motors. You might get lucky and its just a loose wire.
Good luck! 👍
