So you seem to be saying that partial shading doesn't lower the output voltage if you are using PWM?
For PWM, the panel voltage needs to be close - but a little above - the battery voltage. So if partial shading lowers the output voltage of the PV, it seems to me that it would cause the the voltage from the panels to go too low for the PWM CC to work effectively.
But, if you are really saying that the partial shading only lowers the voltage for MPPT controllers, maybe you can explain how the PV panels know that you are using PWM vs MPPT.