You still know more about this bike than anyone else
Your tests indicate that the bike is charging when the lights are off but it is not charging when the electrical load is higher, with the headlight on.
As Jens says, maybe the RR is failing.
Answers nearly always generate more questions!
Do you have a wiring diagram for this bike?
How is the condition of the wiring loom?
Are all the earth/ground connections good?
Is the battery a known quantity - a good 'un?
Have you changed anything recently?
(An electrical fault might be the result of a particular single event or a gradual deterioration in the loom or a component).
Have a read through this thread for some ideas about fault finding:-
The beginning and the end are most relevant, unless your loom is as bad as the one discussed in that thread.
You need to be logical with mechanical problems and even more logical in faults of electrical circuits.