Don’t know for sure though an instinct of mine would be possibly someone over at Mercury marine may happen to know of an issue such as this in the past to where they may even have some sort of documentation to something like in the archives.
Using your engine serial number and a description of the symptoms, maybe try sending Mercury customer support via their website a message and see if they respond with something helpful or not.
Manufacturers of outboard motors and MerCruiser inboard engines, with over 4000 dealers in the United States.
www.mercurymarine.com
What is your engine serial number?
What value numbers are you reading on the multimeter?
Are all the little ground wiring throughout the engine and battery cables are clean and tight at both ends?
Any heat to the wiring/s?