Slur curvature is hard-coded (fixed control points), not derived from stem direction — a latent defect, not yet triggered because slurs are voice-1-only

A slur on a shared stave is conventionally curved on the side AWAY from the stems (curving below the noteheads when stems point up, above when stems point down) so it doesn't collide with the stem or beam.

A slur curving TOWARD the stems instead of away from them — colliding with the stem, beam, or the opposite voice's notes.

Gould, Behind Bars, and Read, Music Notation, ch. on slurs and ties: curvature direction follows stem direction on multi-voice staves specifically to avoid stem/beam collision.

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