2003 Mazda6 transmission

I have a 2003 Mazda6, 108K miles. Had the ATF changed last December at the dealer. Prior to this, there was some hesitancy to shift into 1st when cold. Last several months, it has gotten worse. Engine revs (as much as I allow it, which ain’t much) and then goes into first. Dealer thought they had overfilled the ATF, and reflushed and refilled. Said they couldn’t find any other problems. There is no CEL. Problem was better for a short while after fluid change, but now back to long hesitation. Car goes into Reverse fine when cold. Once the car is warm, I don’t notice the hesitancy to shift to first.

Any thoughts on what to look for or how to diagnose?

An update. When using the manual mode of the ATF, the car appears to shift to 2nd gear before finally going into 1st.