3 days ago I got my Mercury mariner 2007 4Wd 3.0 120,000 km back from the transmission shop, the first day I got it back it started having problems shifting from 2nd to first in stop and go traffic, I took it back to the shop and the owner said they forgot to reset the TCM after they finish with the transmission, he apologized and reset the TCM, then get took it out on a drive with the scanner attached and all was good, I got it back and it shifts a lot better now, it will however occasionally and very rarely hesitate while shifting which according to the tech is normal and related to the learning process, my main question is Is there a breaking period for the transmission ? I want to take it out for a 300 mile drive in a few days and want to figure out if it's ok to be driven in the highway, the other question is about the learning process of the transmission, Is this a thing or is the tech blowing up smoke ? It all sounds legit to me but figured a second opinion wouldn't hurt

Thanks for the help !