My experience is that a 3 position switch works best. These cameras typically provide for 2 modes, video and photo on the same PWM input, i.e. moving the switch low sets the video mode and starts video recording, moving the switch high sets photo mode and takes a photo. In the neutral position the camera is idle.
I assume you only want photo mode if you are using the FC to control the shutter, so you may need to adjust your PWM values to go from neutral to high, and never “low” if that makes sense. I would try a value around 1500 for your low value in shutter settings, and 1800 for your high and see if that works.
With your current PWM values, your camera is likely defaulting to the “low” mode, whatever that is, as soon as the flight controller boots up.
I would also examine the “default” startup mode for the camera. These can usually be set to video or photo. Also, make sure you have turned off any option in the camera to start recording on power up.