Greetings!
I've had the following issue for years & I cannot find any solution to this nasty problem so I've decided to send this out to the Universe in hopes that someone can help me solve this.
 
I first noticed this problem years ago on a piece of software called Garritan Stradivari but recently noticed it is also evident on IK Philharmonik, Native Instruments Symphony Series String Ensemble, and Bandstand to name a few. Even my Roland i7 Integra (hardware) performs in exactly the same way when I access a String Section or a Violin solo.
 
I think the technical description for what I am experiencing would be referred to as a crescendo, but I refer to it as BLOOMING. Whenever I play a note or chord it starts out rather quietly then seems to increase in volume as I hold the note or chord. I do not increase the pressure on the keyboard as I play so I don't invoke Key Aftertouch or Channel Aftertouch in an attempt to add vibrato or modulation.
 
When I record the audio it always appears similar to a sawtooth wave where it starts to rise then when it peaks the fade drops rather quickly at a very steep angle.
 
Any help or opinions on this topic would be greatly appreciated.
 

The first thing that comes to my mind is a radical velocity-curve setting in your keyboard. Allthough, I've never heard that a velocity-curve could be as disturbing as described.

What happens if you import a midi file and play it back, rather than using your controller to trigger the notes?
 
Bob Bone