A flag hooks toward the notehead side matching its stem's direction — Up-variant glyph for up-stems, Down-variant for down-stems

SMuFL defines separate Up and Down glyph variants for every flagged duration (e.g. `flag8thUp` vs `flag8thDown`); the flag is not a single symmetric glyph reused for both directions — an up-stem note must render the Up variant hooking down-and-right from the stem tip, a down-stem note the Down variant hooking up-and-left.

A stem-down note rendering the Up-variant flag glyph (or vice versa) — this would only occur from a stem-direction/glyph-selection mismatch, most plausibly as a downstream symptom of a stem-direction bug (e.g. the note's stem_direction is correct at render time but the glyph selection reads a stale or independently-computed direction).

SMuFL glyph naming convention (paired Up/Down glyphs for every flagged duration); Gould, Behind Bars, flags section; Read, Music Notation.

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