Mine does this as well when cold. Doesn't matter if overnight or a week it still delays going into reverse a few seconds. Also delay shifts on the 3-5 shift when cold too. I just chalk it up to another quirk of the 10 speed..

 

If I have the 3-5 issue, it is so minor that I don't notice. This issue is obvious, but if it isn't an indication of something yet to come, I'm fine with it. This 10 speed has been excellent.

 

How about accurately checking the transmission oil lever first with the transmission fully warmed up to its operating temperature, could take a while to warm up in the winter time. Once you know it's full of oil, I might suggest its the torque converter's oil draining down over a period of time (they don't normally drain down) and it's taking the system a few moments to refill the converter with oil. It's certinally not good for transmission clutches or bands to be slipping, especially under load but as the one member suggests his vehicle did it new and was still doing it at 77,000 miles later when it was traded in. A torque converter half full of oil would be enough to meet it lubrication needs but not enough to deliver its fluid torque connection, to drive the vehicle, until its filled. Just a suggestion but not a reason to start changing parts with our further investigation.

 

Come to think of it, I back into my garage. I never cold-start my truck and go into reverse, and now I'm curious lol.