Can someone explain this grounding phenomenon?
I have had bad experience with a ground rod.  I drove a six foot copper pipe into clay soil to ground an antenna and then used an ohmmeter to find its resistance to the house ground system which was connected to the power company and a ground rod at the box. The box was about 60 feet from the antenna.  I was surprised to measure 40 ohms. This happened twice.  As a result I would never use the ground instead of a power return wire.  Not only would I expect to loose power but I would expect the ground rod to be damaged by corrosion.
If the delta power source and system had no ground connection at all I would expect your 3 phase circuit to act like it was open.
The light connected to the tube would likely be dim.