The model cannot distinguish before-the-beat vs on-the-beat grace-note realization

Historically and in modern practice, some grace-note figures are realized BEFORE the beat (borrowing time from the PRECEDING note/silence, common in some 18th-century keyboard "flick" ornaments and in many modern acciaccatura realizations) while others are realized ON the beat (borrowing from the FOLLOWING principal note, the standard Classical appoggiatura realization). These are different rhythmic placements, not merely different notated shapes.

Gould, Behind Bars, ch. Grace Notes, and standard Baroque/Classical performance-practice literature (relevant to Gradus's Bach curriculum) both document the before-the-beat/on-the-beat distinction as a real interpretive fork, not a rendering nuance.

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