Wow. I think the only thing left is the disconnect and/or fuses. Do you read any resistance across any of the fuses? You have to be losing a leg somewhere. I don’t think a ground would cause this. I would definitely get a 3 phase voltage monitor on it. You might be able to catch something at the disconnect with a meter set to Max from line to load, one phase at a time. Any resistance will show up as a voltage drop.