Hm.
Had that sort of thing occur a couple of years back. Shifted into reverse, but the rear-camera view didn't appear. Wouldn't for a few cycles (of drive, reverse). But then it decided to suddenly work, later that day. No idea why it didn't work, or start working. And it hasn't occurred again, since.
Perhaps something in the shifting mechanism, where a sensor doesn't quite function correctly. In my case, I do a bit of shifting in manual mode. Not excessive, but it might well have been enough (this past ~9yrs) to have caused some sensor in there to begin to give up the ghost. (A bit like the brake sensor beginning to go wonky, causing inability to shift the lever out of "P".) Haven't had the console/dash apart, yet, but it might be worth adding this to the "check it out" list, if ever I have need to get in there.
It's always something, eh?
