I load Solo Strings > Violin > Violin 3 Guarn Solo
While playing an ostinati in the lower register, if I hold down a note in the higher register, it stops playing after a bit even when I'm holding it down - this shouldn't happen - Because if I just hold this note down in the higher register (without playing the ostinati) it sustains properly and doesn't suddenly stop sounding.
just a guess, but I suspect note stealing. The polyphony of the solo violin is 4. If you play a second line (the ostinato line) too fast, or with enough overlap, and reach the max polyphony of 4, the ostinato line may "steal" the oldest (sustained) note.
Watch the polyphony count on the player as you exectute your pattern and see if the polyphony reaches 4.