In sig: 2020 3500 Limited HO/AISIN, loaded with pretty everything except sunroof and VSIM.
As I recall, the trailer brake application occurred on both regular cruise and ACC - because they both hold speed but only ACC will slow you more with a slower vehicle in front - but I only had one opportunity for that to happen coming down the eastbound side of Wolf Creek Pass and wasn't playing with it much.
I think
@MikeXM has done a little more testing and confirmed brake application in any cruise mode and any exhaust brake setting.
My preference would be let it run the service brakes on Auto mode, but do not use service brakes when in full ON mode.
Of course, without cruise on, there's no service brake application, only exhaust brake.