The magnetic drive would have no effect on the case ground. It would seem, for some reason, the case of your pump has a connection of some sort with the DC positive wire. Lots of things could cause this from although on a new pump, I would suspect either dampness or an assembly issue.
To avoid stray current issues, I would disconnect the bonding wire from this pump. On the fresh water pump like this there are minimal chances for corrosion.
With the bonding wire disconnected, I would use a volt meter to check the voltage between battery negative and the pump motor housing. If that shows 24volts, then there is a problem with the pump motor.
For what it’s worth, on the latest models of these pumps in the larger sizes, Marco has introduced a remote control panel that shows run status, duplicating the functionality of the original Amel "Pump-running" light, with some other features as well.
Annapolis, MD, USA