Shared-stave stem-role inversion becomes an engraving problem only when SUSTAINED, not on a single occurrence

An occasional voice crossing on a shared stave (the up-stem voice briefly sounding below the down-stem voice) is normal and doesn't need remediation. SUSTAINED or frequent role inversion makes the fixed up/down stem convention actively misleading — the reader starts to see stems pointing the 'wrong way' relative to pitch often enough that the shared-stave stem convention stops doing its job.

A passage where the up-stem voice sits below the down-stem voice for the MAJORITY of a phrase, so a reader can no longer use stem direction to reliably tell which written part is which.

Gould, Behind Bars (fixed stem-role convention) combined with general engraving judgment that a convention's value degrades once its exceptions dominate.

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