Beams break at interior rests by default

A rest interrupts a beamed group unless the engraver deliberately chooses to beam through it (with a stemlet) to preserve a beat's visual grouping — the conservative, safer default is to end the beam at the rest.

N/A as a fault in Gradus today — describing current (correct, conservative) behavior.

Gould, Behind Bars, ch. Beaming, presents beaming-through-a-rest as a deliberate, less common choice (mainly seen in drum/rhythm-section notation) rather than the default; Read, Music Notation, treats breaking at rests as the standard approach for general instrumental/vocal notation.

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