This is very interesting because I've seen similar behavior too. Just some thoughts, no solution yet.

1. Are you playing a downloaded midi file or is this a Biab generated song?

2. Is it only with Kontakt or have you tried playing the same song file using other synths?

3. Are you generating or regenerating some tracks using Biab styles?. When Biab generates a song it will put in CC7's for dynamics and presumably those will change every time a track is regenerated.

4. Is this only with midi tracks or does it happen with RT's or other audio tracks?

5. Does Session strings create string sections and is that what you're using, sections as opposed to single violin parts? Could it be possible that the different parts of a string section will have different volume settings that could be automatically varied by Kontakt for realism? This would be similar to how Biab will slightly vary each regeneration of a song.

I don't know these answers, just throwing stuff out there.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.