Even with a corner ground, I would imagine the voyages will be the same between the legs.
True. However the voltage between each phase to ground would be different. If the return path is ground instead of the other phases, the one working heater would see 400V across it instead of the expected 230V.
A floating 400V delta would see about 230V from each phase to earth. Ground one corner and now two of the phases would measure 400V to earth and the grounded phase would read 0V to earth.
