There are so many things that it could be. I doubt any aircraft has ever produced as much electricity. An airplane is a "floating ground". This many trons running through a machine provides huge opportunities for errors.
Everybody was a hoping for a simple smoking gun. They will find the answer, but when they do, it will probably be a single line of bad code, a single electronic component sourced from a dodgy supplier (China), or something poorly grounded, or grounded in a slightly different place than it should have been.
Almost all the world's lithium comes from China. Maybe somebody added a little "filler" to make a little more for little cost? Maybe a single grounding wire or ground plane has higher resistance than it should? Maybe a single component in the charging and monitoring system reacts poorly to temperature change?
Oh yeah, isn't carbon fiber a good conductor of electricity? Maybe a carbon panel is discharging static electricity into one part of the system, throwing off voltage readings?
Too many possibilities. Good luck to them.