Could be different camera software/resolution (uhm)
It could also be the poor resolution of the screen. I can't comment on the 2017 screen, but the 2016 screen has poor resolution. My phone (S7) has better resolution than my infotainment screen. I even thing my S3mini had better screen resolution than the CX-5.
At night, with the back up lights on, the screen is really grainy and details are not very detailed. I could have sworn, the purpose of the backup camera was to aid in backing up as another source of information and to help the driver see if there was anything immediately behind the vehicle.
Now, if someone is trying to parallel park at night, in the rain, I think there is enough detail, plus the distance lines to give an accurate picture to not hit the car behind you.
I've tried adjusting the screen's brightness/contrast in both the "Day" (lights off) and "Night" (lights on) settings and going up and down with each in the two categories does not really change the results.
I'm sure things would be different if the screen/backup camera were the rear view "mirror". If that were its main function, I'm hoping the screen resolution would be different.
Maybe the camera used just does not do well in low light conditions. Again, Mazda has been accused of cutting costs on certain things in the CX-5 and maybe a low res, poor spec camera is just another cost saving measure.