Well the 8 speed had a lot of issues when the truck was first released in 2015 but 95% of the issues were solved by changing the automatic transmission fluid. There is a TSB out there for GM to replace the transmission fluid for free on these trucks. I'm not sure if your 18 would fall under that but might be worth checking out. This transmission will be gen 2 which supposedly has improvements over the gen 1. I still was hoping for a 10 speed.
Edit: Looks like 18's are covered under the TSB. https://static.nhtsa.gov/odi/tsbs/2019/MC-10163890-9999.pdf