After some more research I can conclude that the electrical system is clearly WIP. Please refer to my findings below and correct me if I'm wrong as I am not an aircraft technician. If not it's not WIP, I hope the testers would be so kind to file an official bug report.
DC VM readings under different conditions:
BEFORE FLIGHT
Everything OK
DURING FLIGHT
Battery (on) - 28V (should be 24V)
Battery (off) - 28V (yes, it's turned off!)
Main Gen (on) - 28V
Main Gen (off and reset) - 28V (Should indicate 0V ofc. Does show 0V when main gen breaks down)
STB Gen (on) - 27V
STB Gen (set to starter) - 27V (Should be 0V)
ESS Bus (Batt, Gen on) - 28V
ESS Bus (Batt off, Gen on) - 28V
ESS Bus (Batt on, Gen off) - 28V (see Battery (on) - should be 24V)
NON-ESS Bus (automatic on) - 0V
NON-ESS Bus (normal on, Batt on, Gen on)) - 28V
If you switch the NON-ESS Bus back to automatic it will read 28V instead of 0V. You can't revert this action