Distortion around C-F above Middle C

Hello, I'm using PTQ 5 Stage trial. Starting at the C above middle C (normally this should be C5, with middle C at C4, but PTQ seems to consider middle C as C3) there seems to be a slight but noticeable amount of distortion. Harmonic, like mild overdrive from a guitar amp. Then as you go up, it gradually decreases over the next 5-6 half steps.

Once you get to G or so, it seems to be fully clean again. And all the notes within middle C's octave are fine. So it's a noticeable shift in quality from the B (B4?) to the C (C5).

Have not checked all of the instruments, but it happens on the main ones. And I've noticed it on 2 different computers/setups, both with speakers and headphones (AKG K240M). But as it's limited to a small range of notes, it should not be an issue with the setup anyway.

Otherwise liking PTQ enough to buy it, but this is enough of an annoyance to keep me on the fence. (That, and the hammers are a little loud (and non-adjustable in Stage?), esp considering my controller makes enough clunk already!)

Thanks for any help!