I have a Kawai upright (CX-5H) and it often happens that, when lifting the fallboard, I notice some keys are raised compared to the others. Once struck, they return to the aligned and correct position. Trying to lift all the keys slightly, by applying light leverage on the outer edge of the key, I see that some return immediately to the aligned position, while others do not drop back and remain slightly raised relative to the keyboard plane. What does this depend on and what kind of intervention would eventually be needed?
Three things: check if the lid presses on some keys. check if the central holes of the keyboard are too wide. Check the general alignment of the keyboard by acting on the thickness of the pins of the center key bar.
Check the entire adjustment of the mechanics
Naturally these are hypotheses, it would be necessary to observe the phenomenon closely
