Stop in for a cup of coffee

only thing i can think of is the ground strap isnt heavy enough.
I don't see the reasoning here.
The rotor brush must be grounding if the regulation is not working.
Try disconnecting the field negative wire, at the alternator.
upload_2019-11-13_17-59-5.png

If the alternator is still producing unregulated power without the return wire, then brush holder or terminal must be grounding.
Does that make sense?