Opaque MassIt seems I've run into this weird EastWest Opus cut-off bug, where it just decides to not sustain all the notes I play. It's reported at https://vi-control.net/community/threads/eastwest-hoopus-notes-cut-off.128457/ and none of the suggested fixes fix it. Super annoying and quite disappointing of such an expensive piece of software.
My solution, should anyone encounter this bug, is to double your Opus midi-bit earlier at 0 volume. Stupid, but it works.