i have the same issue with my casio privia px 555r a few keys are sunken alot more than in your case around 10-15(too lazy to get up and check lol). There were few of them at first now its a whole bunch of them. im searching all over the internet and cant find a reason why this is happening, and it feels like a counterweight problem every key i strike has this double kickback effect in response like u have a weight on a string and jerk up on the string and it becomes taut after two bounces. really weird. dont buy if u cant get someone to fix it or cant fix it yourself for cheap.
