Like you, I have had it to my dealer several times, though the first time or two I don't think they noted it on the service order as I had it there for other issues as well.
Make sure you get it documented from now on and then perhaps let them know you will be pursuing the lemon law. That might make them have a sense of urgency.
2018 GMC Sierra 2500HD, DMAX, CC, 4WD
2" Rough Country Level
Fox Performance 2.0 Shocks
Cooper Discoverer AT3 XLT, 285-75R18
Leer 180 Cap