USE_CONTROLLER_FAN with hardware PWM fan (no replies)

I've come across something weird and wanted to make sure I'm not doing anything stupid before adding an issue on Github.

I'm using an MKS Gen 1.4 board flashed with Marlin-bugfix-2.0. I'm trying to use a 4-wire PWM computer case fan as a fan for the controller case to keep the drivers cool because I was getting overtemp warnings.

I've attached the yellow and black wires to the 12V rail directly and I thought that connecting the blue wire (which is PWM) to the board would give me control over the fan without needing to use a mosfet and softPWM.

In Configuration_adv.h I've set the following:

#define USE_CONTROLLER_FAN
#if ENABLED(USE_CONTROLLER_FAN)
  #define CONTROLLER_FAN_PIN 2           // Set a custom pin for the controller fan
  #define CONTROLLERFAN_SECS 60          // Duration in seconds for the fan to run after all motors are disabled
  #define CONTROLLERFAN_SPEED 100        // 255 == full speed
#endif

But the fan just blows at full speed the second I turn on the board and it doesn't go down again. Ideally it would be off all the time and only turn on when the motor drivers are on.

In addition I've got the extruder cooling fan set to automatic and that works fine, but that's also the reason I don't have any more mosfet outputs on the board.

I just wanted to check that I'm not doing anything that's impossible. Or that I'm using a pin reserved for something else. Or that I have to define the pin somewhere else, like in the pins definition or board definition file. Because the PWM implementation from each manufacturer is different, it might just be that my fan (an Arctic fan) does 100% rpm when the PWM signal is 0; I've seen this before, but wanted to be sure the firmware side is ok before I tackle that.