What would cause a loss of acceleration?

What is your question?

While driving on the highway and slowing down due to traffic when I stepped on the gas the car would not accelerate. I managed to pull off the highway, put the car in park and shut the car off. I restarted the car three times but it wouldn't go anywhere and it did not accelerate, engine remained at or below idle even when I stepped on the gas. The car was towed to a dealership via flatbed. After the car was off the truck, the mechanic was able to drive the car and couldn't identify the issue. All diagnostics are reading normal but the dealership thinks I need a new transmission. They've had the car for more than a month. Lastly, five days before the breakdown, the same dealership had tuned the car up, replaced the battery and changed all the brakes.

What makes this problem better or worse?

The car didn't exhibit any issues before the breakdown.

How long has your 2012 Hyundai Sonata had this problem?

N/A