Open score uses natural pitch-based stems; short score forces stems by voice

When each voice of a polyphonic texture gets its own dedicated staff (open score — traditional fugue engraving), there is no cross-voice disambiguation problem, so stems follow the ordinary pitch-vs-middle-line rule with no forcing. Only when voices are compressed onto a shared staff (short score / SATB) does voice-identity override pitch.

A voice alone on its own staff having stems forced anyway, or conversely a voice sharing a staff NOT having its stems forced.

Gould, Behind Bars, open score vs short score sections — exactly why fugue is traditionally engraved in open score and chorale in short score.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.