Battery is ONLY used to start bike. ALL power consumed after that should be from alternator only. That's because in dual-power parallel circuits, higher voltage one supplies power and lower one receives. So after starting, power sources may look like;
11.9v = battery
13.6v = RR
This means that power will ONLY flow INTO battery (higher voltage dictates flow direction). Battery voltage can never rise above RR. Most it will do is reach same voltage as RR and current stops flowing into battery. Only time power flows out of battery is if somehow RR drops its voltage below battery at later time after charging battery. In which case, it's more likely RR problem because short would manifest itself from very beginning.
Might me good idea to measure amperage draw of light circuit. Higher drain than calculated could be sign of short.
EDIT: wait, non-U.S. bikes can turn off lights. Try lights ON from very beginning. Does voltage start dropping immediately after turning on bike? Where do you have leads for meter connected?
Which RR model are you using? Does it have separate sense/reference line? Where is this connected?
Last edited by DannoXYZ; 8-15-2019 at 10:43 PM.