I’d say the reason is quite simple: you should not use marching cymbals (1 line) unless you use Auto or Marching Band Percussion playback template. I just tried the 1 line and the kit version, and I don’t get the flute, but some conga (which is plain wrong too) with NotePerformer… If I use Auto, then Marching Cymbals Dorico get loaded in Halion Sonic and it’s perfectly fine.
Every time you don’t hear the right instrument, there is a mismatch between the instrument you want and the instrument that’s loaded. It can be solved by changing the instrument in the instrument picker, or by changing the playback template. In the meantime, what we can do is report those mismatches, because the Team can probably solve most of those, provided they know about them!