Hi Skyking,
I see you say you're running the latest versions, just to check.
Kontakt 5, do you have the 5.10.6066 update ?.
Galaxy D also had a 1.2 update that was a major rewrite.
Do you have the repedalling option enabled ?.
I see you're definitely not using continuous CC for sustain for half pedalling, which BTW, is very cool - it's so much more like a real sustain pedal. I hate working without it now for solo piano.
I do recall having some funnies with sustain when I first got Galaxy D, but that was before the 1.2 update which was very much a ground up rewrite (according to their docs).
That was also Kontakt 4, and many updates ago.
I don't recall having problems of late, and I just did a round of full on classical solo work, no glitches at all.
I use both the repedalling option, which does apply to your situation because it doesn't require a continuous CC rather than On/Off sustain, AND the half pedalling with a continuous sustain pedal.
The only other thing that crosses my mind, is with Kontakt options - CPU overload that results in voice stealing or dropping.
Cheers - Cliff