2nd gen 2010 shifting weird between 3 to 4

How’s it going, my 2010 auto is doing a rev up at the top end of 3rd going into 4th. Only does it as the truck is warming to temp. But once at proper temp it shifts fine. I have had a drain and fill done recently. The truck does have 206k miles and I bought the truck with 183k if I remember right. I ended up needing the trans cooler lines replace shortly after purchasing the truck and the tranny was if I remember correctly 6 qts or so low on fluid that was added at the time of trans cooler lines being replaced. What could this be?

 

How long was it down 6 quarts

 

It was probably down 6 quarts at the time maybe a month or so, got it fixed and then about 1 1/2 months ago I had an oil change and drain and fill at the Toyota dealership in Bow, NH where they said the issue maybe because it was a mix of old and new fluid so they drained and put in the required amount to spec via machine and computer.

 

I did have the trans cooler lines and the trans fluid topped off in December 2021 when I bought the truck in the end of October 2021.