Sense wire for alternator

Read the voltage at IGN, is it steady? If so, regulator is working fine.

Varying load on IGN, from ignition may result in some transient behavior. If you have other loads there, that. Is a problem too. Loads should feed off ACC, and relays for large load. The IGN circuit has skinny wire.

Have you measured 0.3V variations from battery to IGN?