While the "flare" may be common in gm transmissions
It is not Normal and should not happen
The 6T40 is an equinox transmission
And Traverse transmission is 6T75
The root cause might be similar.......
..... PI0376A – Flare or slipping on the shift from 2nd or 6th gear – Chevrolet Equinox 6T40
Problem – When this common Equinox transmission problem occurs, the driver may experience slipping and/or a sudden flare/surge when the transmission shifts into or out of 2nd/6th gear.
Solution – This problem is often caused by debris blocking the opening on the channel plate and spacer plate that provides oil to the 2-6 clutch regulator valve. To correct the issue, the channel plate, spacer plate and valve body will have to be cleaned.
If this is the same type of problem ^^^^
And your GM dealership will not fix your problem for you.....
Then you might try, might RISK, a transmission flush
My understanding is
Some transmission flushes will run a "detergent" through your transmission , in an attempt to clean gunk that have lodged themselves in small holes where they block flow....
"Rev up your engines" Scotty on utube says to never use such a flush, because you might plug up a hole in your transmission..... but if you already have issues.....
Or you could try....
As preventative maintenance, as mentioned by "cgostola" ^^^
What some folks do as described on this web site
is change 5 1/4 quarts of transmission fluid using "gravity" to drain transmission fluid. Then replace the fluid with 100 prevent synthetic fluid, this will give you about 50 percent clean new fluid. Then you might repeat this in another 500 miles....
menards sell 100% synthetic gm approved transmission oil for less then $6 per quart.