I have a 2022 refreshed Trail Boss LT that I am having this same trailer brake issue with, along with a couple other issues.
My backup camera drops out intermittently, my wireless charger connects and disconnects over and over without charging any phone I have placed into it, and when I tow any of my trailers that don't have electric brakes(using the 4 pin plug) I get all kinds of error messages.
The dealership gave me the bulletin on the trailer brake issue and told me they can't do anything with it until GM finds a fix. They also found codes for trailer turnsignal issues and a trailer brake gain issue. They assume these are related to the bulletin and want to wait to her more from GM.
As for the backup camera, they couldn't get it to replicate the issue, but they did pull a code out of the computer. They told me that they have to replicate the problem before they can do anything about it. They also said they can't justify a tech sitting in the truck for an extended period of time to get it to replicate the issue. So I guess I just get to deal with it forever, unless it is related to the trailer issue.
They did say that my charging pad is bad but they cant fix that either because the parts are not available.
I did open a case with GM. They have basically told me that I am a guineapig and have to deal with the trailer issues until GM finds a fix. They are unsure why the camera is dropping out but want me to take it back to the dealership as soon as it happens again. I told them that is not feasible because it will not be out when I get to the shop, they said oh I guess your right. Then then told me I have to deal with it until the dealership figures it out. I then asked about my charging pad that is not available. All I got was a sorry but you will get one when they become available, which they have no clue how long that will be.
The backup camera and charging pad are not huge deals, but for a $65000 truck they should work or be fixed in a timely manner. My major issue is the trailer stuff. I bought the truck to haul with, and I can't do that safely. GM doesn't give a crap and It's damn frustrating.
I feel my only option is to trade it in for a truck that I can tow with and take a loss on the one I have only had for a little over two months.