If it's shifting fine at low speed, that's a good sign, but I wouldn't ignore it either. Sometimes issues only show up under load or higher speeds-things like low transmission pressure, worn clutch packs, or sensor/calibration issues can start subtly.
I'd keep an eye on:
How it shifts when hauling or going uphill
Any delay, slipping, or hard shifts
Transmission fluid level and condition (burnt smell = red flag)