I tend to believe the flare problem is not entirely transmission software related. In 3 years and 30k miles of owning an '18, I've never experienced a flare issue.I just picked up a used 2017 SLT Acadia with 35k miles and found the 2-3 flare. Never noticed it in the test drive. Oddly enough in my commute there are two left turns on level ground where the flare is very evident and feels to be around 1/2s or so, enough to feel the vehicle to slow. I'd be interested if flashing solves or minimizes the issue.
In my mind, I think it is a combination of things, driver style, transmission software and cylinder deactivation software.
Some owners complain about it, but does not seem to be a wide spread issue.