Another bad voltage regulator?

Yeh you are headed there. As I keep harping LOL the ground at the VR flange MUST be same as battery neg and the VR IGN terminal MUST be same as battery POS. That is really the secret of the whole deal

"In part" that is why I caution against "one wire" alternators. They are OK if the charge wire--which is the sense wire--is oversized enough to keep sensing correct. A separate sense wire is by far better