I am proposing this issue regarding a C five octaves up on my upright piano. The video below for a few seconds gives an idea of this echo I hear when releasing the key.
It seems like a problem with the damper regulation. However, I wanted to get your opinion.
Try to see if any part or any bass damper or of another area remains detached (and creates resonance). Also check the regulation of the sustain pedal. It could be that it is poorly adjusted, without play, and causes some dampers to remain imperceptibly raised
Thank you Maestro, I solved it; it was exactly as you suggested by acting on the damper regulation screw.
I should have thought of it sooner... It was the F# two octaves below (F#3). In fact, the third partial of the F#3 is C#5, which is the note that was causing this echo.