My 919 recently started wobbling the handlebars at 40-45mph. It only happens when I take my hands off the bars. When I put my hands back on, it stops. It happens very consistently at this speed.
There is a natural tendency for this to be able to occur.
Typically, it is not a precursor to highway speed tank slappers.
Typically, it's nothing to be scared of.
BUT, that is not to say it should be ignored.
IF everything has been checked and found to be OK, and I do mean everything, then I'd suggest revisiting the steering head bearings again as they are a likely suspect, as CB700S pointed out.
Steering head bearings in anything less than perfect condition should be binned.
Steering head bearings are as problematic when they are set too stiff, as when set too loose.
The factory manual setting is too stiff for my liking, but it can take many tries to get them set looser with no detectable clearance.
Never set them with detectable clearance.
Aged grease can be a problem, but it very depends on the grease.
I've seen grease a few years old losing its oil, as well as decades old grease that is just as fresh behaving as when it was new.