The one time mine went black, it did the same exact thing. Literally same thing, open door and it flicked on for a second then back to black, backup camera worked but no grid lines. I took mine in but they didn’t do anything because it didn’t have issues while they had it. Only thing that has reliably fixed it for me is turning the truck off for more than 2 full minutes. Although it only the did the black screen thing once, the rest of the times were just the touch screen not responding to touch. I’m pretty convinced it can’t be a loose connection because like you said it never happens while driving, once it’s working it continues to work. It’s only on start up that the dice is rolled to see if it works.