Why did the MPPT charge controller struggle to work?
The MPPT controller itself introduces more voltage drop into the circuit due to the increase in complexity. It also needs more “voltage-overhead” to operate than the PWM controller due to the ‘switching’ circuit within the MPPT controller. So when the voltage drop along the cables becomes significant the MPPT cannot function as expected.
When the voltage overhead is not high enough for the MPPT to function correctly it reverts to PWM operation.
The MPPT controller also reverts to PWM operation when in absorption or float modes. During these two important charge modes the controller must reduce the current to the battery to avoid overcharging. So in these critical stages of charging the MPPT function has no benefit.
https://sunworks.co.uk/improve-mppt-charge-controllers