First post here- signed on due to looking for tips on this exact same issue. I just drove the truck home (2019 Sierra SLT) from the dealer yesterday, today was our 1st full day with it.
Issue on mine is the rear passenger door as well, I need to unlock repeatedly 3-4 times before the unlock pin will slide all the way up. It seems like the pin is getting stuck by a tight fit. Keyfob, door unlock button, etc all to the same effect. All other doors operate flawlessly.
Thinking back, I noticed this door not opening at the dealer yesterday, but thought I had just pressed the keyfob incorrectly and shrugged it off as it eventually opened.
Minor annoyance that I'll need to head back to have them fix an issue on a brand new vehicle. (On top of this, the check engine light came on today while driving too; haven't even put 50 miles on it yet...) Aside from these annoyances, truck is fantastic.