Under a melisma slur, vocal beaming conventions differ from instrumental beaming, and Gradus has no vocal-specific beam logic

In vocal music, notes under a single syllable are conventionally beamed together even if that beaming crosses a metrical grouping boundary that instrumental beaming would respect; conversely, a syllable change conventionally BREAKS a beam group even mid-metrical-unit. This is governed by text underlay, not meter.

Gould's vocal notation chapter and Read document syllable-driven (not meter-driven) beaming as standard for vocal parts, in contrast to `Beam.getDefaultBeamGroups`'s purely metric grouping.

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