I'm not familiar with your system network but a few Items spring to mind.

1. A motors back EMF is what reduces the full load current and the is affected by magnetism.
1.1 The balance of Magnetism is affected by voltage levels and phase angles as well as supply impedance of each phase.
2. If there are electric railways on the network there are large 2 phase loads that can affect the levels of voltage and the angle.
3. If Eskom uses Static Var compensation to balance and regulate voltages due to large 2 phase loads and fluctuating loads, there is the possibility of phase shift as the thyristor control changes the Vars individually per phase.
4. If you have a low fault level at the point of use, all these factors are aggravated.
4.1. fault levels are often checked by the protection department of Eskom. you can try enquire if the tested individual phases for the phase to earth faults and what the readings were
5. a good practice is to log all large motors with a logger to get a base condition, then relog after any rewind, comparing voltage angles as well.