In your image note that the only cam with rain streaks are the ones in IR...the color image has none. If this issue is important enough to you to resolve, then read further.
I'd say if there's not enough ambient light to force cams into color mode then place outboard IR emitters away from the face of the cam and in such a way as to target the ground that the cam is pointed at; the IR will still be seen by the cam if it's strong enough and oriented properly. Under the cam would be a good place to start with the location of the outboard IR emitters.
The IR emitters should have their own built-in photoelectric cell that will turn the unit on a dusk, off at dawn. Most will also require 12VDC to operate.
You'd then disable the cam's IR so it won't be hitting the rain which is just a few inches away from the cam's face, causing the issue.