3 phase unbalanced lighting system
If you have no neutral in the circuit, not even at the station transformer —true triangle circuit—, the hot-to-ground voltages are arbitrary. The whole circuit is floating in respect to neutral/protective ground.

I expect the voltages you measure depend on the light's EMI circuit's Y capacitors which connect hot1/hot2 to protective ground. This also explains why they change with hot1/hot2 being connected to the station transformer or not.

To avoid this floating, one could tie a leg of the triangle to ground. But that's something the design engineer had to decide, not something to be changed later. It requires some thought about the protective equipment used.