One of my work trucks did that.
Check the fluid with the tranny warmed up, the truck in Neutral, engine running.
An employee was checking the fluid with the truck running and in park. 4 quarts of tranny fluid and it has been shifting like new for 2 years now. Truck is a "98 with 132K on it, has pulled a trailer almost daily since new and has plowed snow for 13 winters.