A fermata over a rest must render — Model A currently drops it silently

A fermata over a rest (holding a silence, very common at the end of a phrase or before a final chord) is standard notation.

Gould, Behind Bars, ch. Bar Rests / Fermatas, shows fermata-over-rest explicitly as a normal case, not an edge case.

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