I had the same problem , adn it was the D too, the one an octave above the middle C as well.
Here's how to fix it without buying anything.
U're going to have to OPEN the piano, i know there's a sticker that says that you should'nt , however it's super mega easy, and you won't harm the piano.
So open the cover, and basically take everything that's out of the way out . ( don't worry it's very easy to reassamble clavinovas, trust me) . the reason that D key stays depressed when pushed is because Either, It's slightly broken or the connection to the hammer is a bit Crooked. Either way, all u have to do is Just take that D key and replace it with another D Or G key. preferable in the lower register wher you might not use those as much.
I had exactly 2 D keys that stayed depressed , I was so ANNOyed and so pissed, it was impossible to practice. I opened it up and me and my friend fixed it.
I believe this case happens alot with yamahas, Sometimes you get a good one, sometimes a defective one. It's all a gamble.
Good luck, let mek now how it turns out.