I recently purchased my first digital piano, a Yamaha YDP-223. I've noticed an annoying (at least to me) ringing sound in the GP1 voice when some of the notes in the 5th octave are sustained. F#, G, and G# are the worst offenders. The ringing occurs whether I use headphones or speakers. I called Yamaha service. They said I'm hearing the sound as it was sampled.

Is this ringing the normal sound produced by an acoustic grand? Do other Yamaha digitals have this same characteristic?

"ringing" is very subjective, so it's hard to say without actually hearing it.

But I am willing to bet it is not a defect.
I know I was super critical when I purchased my digital. I have a Yamaho S90es. It sounded so awesome in the store, but when I brought mine home, I started to notice all the naunces which at first I wondered if they were defects.

IMO, chances are those offenders will not offend you in the context of normal playing.

I know on the S90es, if you turn off the reverb, without pressing the sustain pedal, starting on the 72nd key (G#) the notes slightly sustain after the key is released. Apparently this is how a grand behaves.

Welcome to the forum and enjoy your new DP!!

True: On a grand piano, there are no Dampers on the upper register keys so the note(s) will decay naturally.