I don't think your battery issue has anything to do with your bike in particular because others and myself have experienced similar behaviour from the GSM motor and smaller batteries.
I got around the problem by gearing down. I also don't have that issue with a 20 Ah LiPo battery. Both combined mean that I no longer have exaggerated battery sag.
I got around the problem by gearing down. I also don't have that issue with a 20 Ah LiPo battery. Both combined mean that I no longer have exaggerated battery sag.