Could the smaller rear tire on the touring models act as a 'circuit breaker'? ie the tire spins before there is enough torque to force the gears apart?
@DEcosse posted a treatise a while back about how the design of the Triumph system has the stator working at maximum output continuously, and by changing out the rectifier/regulator to a series model, the stator only works as needed. If you make the decision to go forward, and you have the engine open anyway, you could replace the stator, and if doing so, I would recommend going with the series R/R (plug and play, though the location may need adjustment).
Again going back to the sixties, my 69' Kawasaki 500 triple uses an excited field alternator for the same reason. I stand by my belief that "new" isn't always "improved".
Both of those stator pics (on this page) look good.
It is quite 'normal' for even the pics of the one with some darker coloured resin - that happens because it is running extremely hot and those particular windings just see less oil cooling than those who do not darken.
(ALL of the windings see exact same current, so it is absolutely a function of the cooling that affects some vs the others)
I can tell you quite categorically that a bad stator will be a LOT more obviously burned, not simply discoloured!
* The other place a potential short can happen is at the clamp
The very simple acid-test for stator is just to measure resistance from ANY ONE of the three terminals to the core of the stator - or if still installed in the bike, simply to engine ground.
If it is short (zero ohms), the winding has burned through and needs to be replaced.
If that reading is 'open' (infinite) then it is 'good'
Don't see much wrong with this one either - a bit darker but still nothing obviously burned.
Do the Isolation test described above to confirm but I'm betting is just fine.