Default rest positions for two voices on one stave must be pre-separated

Two voices sharing a stave both default a rest to the stave's middle line; if left alone they stack. Standard practice (and Gradus's own fix) pushes the upper voice's rest up and the lower voice's rest down along the same axis the forced stem direction already separates them on.

A quarter rest in the alto and a quarter rest in the soprano at the same onset both render on the B4 space and overlap exactly.

Gould, Behind Bars, ch. 'Rests', general multi-voice rest placement convention; Ross, The Art of Music Engraving, ch. on part-combining.

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