I agree. Its super annoying tech and needs serious work to make it an actually helpful function. I’ve had the same scenario the OP described happen as well though not with a GM product.
In April of 2019 my mom and I had flown back to New England to head up to Vermont to put together my grams memorial. It was already a stressful and uneasy trip since my mom was constantly on the phone and I had to do all the driving. pretty sure I drove more in that 10 day stretch than I had in 4 months time at home and I was exhausted from it. Anyways we had picked up a rental down in natich, mass where we saw our other family first. Enterprise had put us in a Kia soul and my mom hates those. So she got enterprise to upgrade us and they put us in a brand new tacoma quad cab that only had like 1,000 miles on it. I had not driven anything that new in a while and wasn’t aware that it only had adaptive cruise control and not regular cruise. So as I’m driving from Massachusetts up to Vermont, I forget where we were, I was getting off the main interstate, to go to a smaller side interstate. The off ramp was long and merged into the road with a stop light right where the merge happened into the main road. I had already hit the brakes and slowed from highway speed, and As I came down the off ramp the light turned green so I set the cruise control down to the 50mph limit on the smaller road and I was in the right hand lane to go through the light and keep going down the road. There was a dump truck in the left lane, just over the lines waiting to turn left across the intersection. as I came behind but still along side the dump truck the adaptive cruise slammed on the brakes, hard enough to go from 50 to like 10mph in a very short space. I’m like “what THE HECK just happened….” I couldn’t believe it was so finnicky that the adaptive cruise thought I was going to hit the dump truck. Thank god there was no one behind us because if there was it would have been a really bad accident. I immediately turned it off but then I was pissed because there was no regular cruise control to use and every time I tried using the adaptive on the longer interstate drives I was constantly having to monitor it to keep it from slowing down anytime someone moved into my lane going faster than I was and it wasn’t necessary to slow down. Needless to say when I got home after 10 days I was thrilled to be back in my old ass 2006 Denali that doesn’t have any of that lol.