I don’t know what to say. I checked all the wires in the setup and found a ground wire with a very high resistance. I tracked the resistance down to inside of a factory molded connector. It is a connector that allows to quickly swap out the starter battery charging cable for one that I can use to connect the freediver battery to. I replaced it and now the setup works as intended.
Valuable lesson for me that even if a set of wires is ripped out of plastic new, doesn’t mean its good.
Thanks for your help.