If Boeing can't pinpoint the cause, then it's time to try and replicate the problem.Get a fully instrumented, fully loaded plane of paid volunteers running all electrical systems and circle an airport at altitude for several hours. Pick an airport where the plane is capable of landing quickly. Land and pull the batteries and look at all the electrical system data. Repeat until the cause is conclusive.
Even safer if this test can be performed on the ground, with fire trucks standing by.
Boeing is an airframe manufacturer. They make the wings and fuselage. Major components like engines, landing gear, tires, brakes, air conditioning, etc. are made by companies that specialize making these things, as they have for many years. It's up to Boeing to install these components on the airframe. They are responsible when anything doesn't work. I'm sure Boeing outsources the coffee machines, too.