Yamaha CS80

Hi all
Time for a tech question , hope someone could shed some light on it.
While getting some values to check the calibration of the TRG boards I stumbled into a mysterious thing into the CS80. I got same values from different CS80s so I’m trying to understand something.
That is, if velocity = 10 and the attack time=1ms, I can clearly see a kind of a “spike” at the note-on ; this spike has a constant timing at about 2 milliseconds, and a  constant ratio of about 7db/ 2.99V approximately compared to the velocity level, that is if the velocity gets higher, the spike gets higher amplitude coherently with the velocity level.
This is what I know: what I’m trying to get is:  why does this spike disappear when the attack is slower than 1ms ?
Can anybody show me if is this a flaw or a by design choice?
I’ll attach a graph in the files section
Thanks to anybody who can hint me about that issue.
Max

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