'99 Tportage transmission problem

My wife's '99 2WD Sportage has a delay in shifting when vehicle sits overnight. Once warmed up, and throughout the day, shifting is normal. Fluid level is fine. Thoughts...comments!!!

One can think some seal or O-ring gets stuck when cold and after several hours at rest. Something like this happens with mine: when cold, once car shifts in 3rd, there is absolutely no engine braking when releasing throttle...just like it were in neutral...but as soon as heated up, all returns to normal.
I class this as a slight anoyance...may be some "transmission treatment" additive might solve that, but I consider this class of remedies as worse than the problem...so ...I disregard......