While it may seem like a side track with the internal regulator discussion, the point to be taken is that it may be that the "I" terminal of the solid state regulator needs full voltage to switch on and flash the field to get the alternator/regulator working. The "turn on" voltage my be just marginal and thus worked originally but now doesn't.
If you know the application of the regulator being used, maybe a schematic can be found to verify
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