Grace note(s) should be slurred to their principal note — currently drawn on neither surface

A grace note or grace group is conventionally connected to its principal note with a small slur, showing the ornament leads melodically into the main note.

Gould, Behind Bars, ch. Grace Notes, shows the slur-to-principal as the default appearance (the rare exception being multiple short acciaccaturas in a fast passage, sometimes left unslurred); Ross, The Art of Music Engraving, agrees this is the expected default.

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