ok, let's see... dual field alts get both brushes isolated, one for constant positive the other one for regulated negative. Hence both needs to be isolated.

positive ( blue ) comes straight from ign switch.

negative ( green ) is sourced by the regulator.

This is a non brain deal: an alt sourcing is somehow getting both fields to still work. With regulator unplugged, it is getting still both fields on whichever form. If blue wire is still plugged to field and green is unplugged and STILL is feeding, something is shorted to ground feeding on rotor.

Once I got 18 to 24 volts reading ( depending on RPMs ) and discharge on ammeter. After schatch my head for couple of hours just found vent vanes were getting some friction with diodes leads coming from stator while spining. Just bent a bit the stator leads to diodes bank and done. Maybe a diff problem than you but meaning with this a fail like this is a straight or internal short, somehow.

Now about ohms testing... allong the years the rotor ohms reading changed a bit from 12 to 7 ohms between leads. However no matter what, it must be isolated from chassis if reading straight to rotor tracks. Now, reading between brushes, a single field will be grounded JUST via the grounded brush, but on dual field alts they still must be show non continuity with chassis, no matter what.

If you get ground continuity between brushes, something is shorted to ground. It can be brushes itself or rotor ( I have got maybe half of dozen damaged rotor, is my main fail ). HOWEVER a Rotor grounded can get two kind of damages: continuty on windings and still shorted to ground OR broken wire and one of the leads shorted to ground. Both will cause diff fails. I have not analized that, but MY THOUGHT is the first case can keep still working but full fielding... second case not ( that's for sure ). MAYBE you are getting the first case ?

A damaged diode will cause still nice voltage but not enough charge

many other fails can appear on alts, but I have not analized everyone

on a note appart... IF the brush the green wire was connected is grounded YOU GOT LUCK the blue wire is/was not conected there because that will burn the wire up to ign switch and even blown the fuse link, burning all around.