REAPER may not expose the parent project pointer during plugin creation/setState, so the instrument would silently resolve no sample roots. Only that null-parent case now falls back to EnumProjects(-1); a resolved-but-unlisted or unsaved project still resolves nothing.
sampleRoots matches getReaperParent(3) among EnumProjects' open projects; every instrument sample read goes through an ordered root list (one root today) so a later temp-bank root slots in after it.
daniel