I received my SR+(still HW 2.5) mid-June 2019 and it didn't really do this my first 2 weeks and after I purchaed FSD it seemed to start, but it could just be a coincidence.
An MCU reboot usually helps it load pretty consistently for a few days but I've had it happen same day as an MCU reboot too.
It seems as though everytime our car goes into sleep, it clears some sort of buffer. What I generally do is tap on the rear camera view as soon as I get in my car. Once it has loaded the stream, it seems to come back instantly during that drive.
If I let my car sleep again, it's hit or miss, almost 50/50 during the next drive.
The only consistency I've noticed is if I start up the camera app first thing, it takes a good 10-12 seconds sometimes but it always comes back quickly during that drive.
So if you plan to amaze others with your Tesla, maybe start up that camera app ahead of time for that first impression
I'm on 2019.32.12.2 ATM and every version since initial purchase has had this random bug.
Tesla has detailed the FSD computer architecture quite well, but I don't know much about HW 2.5 other than the specs on Wikipedia and that it has two nVidia Drive PX 2 AI nodes and Tesla is using at least 1 of the nodes & backup camera for the perception NN.
Maybe Tesla prioritizes processing of the backup camera stream for logging/learning sometimes and the MCU is just waiting to get the access back for the driver's display?