I honestly don't know what's causing the issue, but having worked with several VSTis in Reaper (including JABB), I find it easiest to use one Aria instance per track instead of routing 16 different instruments. The Aria plugin isn't too RAM hungry, so 16 instances of it shouldn't hog too many resources. Whatever ram you lose may be compensated with fewer routing errors.
I know that's not the exact response you were looking for, but it may be a better way to avoid these kinds of issues.
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