Automatic transmission have changed a lot, especially in the past 10 years. 3 or 4 speed automatic transmissions used to be common, but now they tend to have 7 speeds at least, some even more. This is done mostly to improve emissions and mpg. A side effect of this transmission design change is there are more frequent gear changes, and at speeds where a gear change didn’t used to occur.
Another possibility is your vehicle has a CVT transmission. These tend to cause the engine to make a rev’ing sound. That’s normal. A friend of mine who had recently purchased a new car w/CVT complained it sounded too much like a race car. But that’s just how CVT equipped vehicles sound.
Another idea, known vehicle problems can be associated w/ a manufacturer’s technical service bulletin. (TSB). TSB’s can take the form of a product recall, customer interest bulletin, or a technical description geared to mechanics who do the diagnosis & repair work. Ask your shop if there’s any TSB’s associated w/this problem.