I had the same issue with a YDP-113 that probably had the same action. I took the whole thing apart and didn't find anything visibly wrong, so I don't think I'd go the "exploratory surgery" route. It's possible that it needs a re-lube, but it could be other things as well. Seems like I heard that hairline cracks in the keys themselves can cause this problem. I never worked out my YDP's issues because I sold it "as-is" a few months later.
Since the piano is no longer under warranty, there wouldn't be any harm in asking a tech what the problem might be. Maybe something you could fix, maybe not, but it would be good to know beforehand.
Come to think of it, a friend just bought a P-80 with the intention of retrofitting it in to an old birdcage upright cabinet, and it has the same issue. It's a very slight problem and only a player who knows how it's supposed to feel will notice it, but I would say that given this and my previous experiences, it's not that uncommon of a problem.
