Mazda 6 ATX

I have a 2004 Mazda 6s wagon with a 5 spd auto transmission. About 1 year ago I started noticing a hard shift between gear 2 and 3. I took it to the dealer for a once-over and they found nothing wrong with the car and no codes. Their only suggestion was that I was using the wrong ATX fluid and needed to get a full-system flush. I had been using Mercon-V fluid because the manual calls for M-V fluid. The Mazda dealer told me that M-V stands for Mazda-V, which is a special fluid that I can only buy there (they even had a Mazda bulliten to prove it). So, instead of getting the $200 flush, I bought 12 quarts of the Mazda fluid and proceeded to do 4 different drain and re-fills on my own, thinking that this would replace most of the fluid. I did not noticed any change in the transmission behaviour.

After that, I went to a local transmission place where he test-drove the car and suggested that I just live with it. He thought that maybe there was a piece of dirt causing one of the solenoid valves to behave abnormally, but said that trying to fix it without a code would be an expensive exploration.

So, my question is, should I be worried about this? The transmission acts and shifts normally for all other gears. And when it does shift hard between 2 and 3, there’s no noise, just a jolt that feels like I was driving a manual and had let the clutch out instantly when the engine rpms were higher than the axle.