All notes under one beam share a single stem direction, computed from the whole group

A beam is a single connected line; every stem it touches must point the same way. Mixed up/down stems under one beam is not a permitted convention in common-practice notation. Beyond simple uniformity, Gould and Ross both describe the beam-level decision as taking the group's overall position relative to the middle line, not any single note's.

A beamed group with per-note stem directions differing — most likely a downstream consequence of a future edit that stops trusting the beam's own group-level recompute.

Gould, Behind Bars, Beams chapter (beam-and-stem-direction unity is a hard rule); Ross, ch. on beaming, same.

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