I rarely play notes above C7 for playing music, but recently did so as part of chords inversions exercise. The top notes on my new Roland 90X piano sounded way out of tune (very sharp). The acoustic piano sounds better, but the high notes sound sharp. I took a few hours looking into it.
The Roland offers the ability to tune individual notes. Roland chose good piano tuners, but some high notes are sharp by more then 30 cents! Looking on the web and YouTube (piano stretch tunning), I learned that all modern pianos are tuned in a similar way. I read three different explanations for it. Two of them make sense to me.
Both explanations point out that tuning a high note to the expected correct pitch will cause issues (a beat) interacting with the harmonics due to lower strings. There is no such thing as “perfect tuning”. It is always a compromise that ends up with sharp high notes. The lowest notes are tuned flat, but less aggressively than the high notes. Different pianos require different number of compromises.
The goal may have been to have the instrument cover the whole range, from bass to piccolo. If it were up to me, I would remove the top octave from the piano (don’t worry, it will not happen soon). That is where the pitch errors are really bad.
It may be that I notice the issue more because of my old age. I no longer hear the high harmonics, and what is left is the sharpened pitch. I asked a couple of younger musicians, they hear it on their pianos.