hi
jwyvern described the phenomenon very well "...The hammer attack is attenuated and initial brilliance lost compared with other notes..."
The problem of the G is revealed only with the vibrato voice, even trying the Tyros 5 vibrato the same thing happens. With all other voice... brass, guitar, celestia, sax etc the phenomenon of the G disappears.
Conclusion It is not a problem with the button, nor with the grease nor with the felt pad.
that would have been solved only with of the assistance, it is not a problem of your sx900, it applies to everyone if it is a good or a bad depends on us;)
It may be a sampling error, but it is impossible for Yamaha to have failed in this. Michael's explanation is very plausible. And I agree with Toril, we will make it a reason, with the loud volume, the style in play, the OTS, and the other 2 voices, I challenge anyone to notice the problem, if it is a problem.
vibrato is not the sound I use and I probably never would have noticed ...
Pietro