The old genny regulators have at least two relays; one to limit the
max output voltage, and the other to limit the max output current.
I'm guessing that your current cutout relay is either welded closed,
or someone has dicked with it, and it never opens.
However, the Voltage cutout relay is working correctly at 14V.

How's this for a scenario: at high RPM, the bus (battery) voltage is
near 14V, so the voltage cutout senses the bus voltage (correctly),
causing the generator to cycle on/off, thereby limiting the average
current to less than it takes to trip your Gen breaker.

As you slow the engine, the battery voltage sags below the level which
causes the voltage sense to cutoff, leaving the generator to deliver
whatever current it can at that rpm, which may still be in excess of
what the Gen breaker can carry. If the current cutout relay was working
properly, it would sense the excessive current,

This is starting to make sense.
The last question is (and it makes sense that it should still work this way)
what part of the circuit does the main power shutoff switch kill? Does it cut
out the current circuit as well as the aircraft power supply solenoid? It seems
to me that if the current ("antibackfeed") contacts were welded closed then
the circuit breaker would pop when the main power switch was pulled on even
with the engine off (it doesn't). Any thoughts here? (I'm not the type of
person who 'throws' parts at a problem until its fixed, unless someone else is
paying:-))