This is not normal but it can happen. When running the engine and alternator is charging the engine battery. The engine battery will be above 13V. When the engine battery reaches 13V about 90 seconds later the Auto charge relay closes and joins the house bank and thruster bank. All batteries are paralleled. The thrusters are large amp draw items when used. This will cause a quick voltage drop. The Garmin electronics is very sensitive to voltage drops. The low voltage warning will alarm because of the the quick voltage drop but will quickly go away when not using the thrusters. The actual voltage at the batteries can be at spec and this can still happen. I would still inspect the condition of the house and thruster bank batteries and connections to confirm this is not causing an increased voltage drop from high resistance or compromised batteries.