IMO, an alternator isn't fed. It is it which sends current to the battery, not the contrary.
I think that disconnecting the terminals or removing the belt causes the same effect : preventing the ALT to send current into the circuit. Am I wrong? I don't know... @Dana67Dart will tell us.
I just figured that if you had an issue in the field circuit (fed circuit), that may be causing your anomalies. Disconnecting those would take the field circuit out of play.