An anacrusis/pickup measure shifts where the true beat grid starts, but the beamer doesn't know that

Beat-group cycling for beaming purposes must start counting from the beginning of the first FULL measure, with the pickup notes grouped according to their own (usually shorter) partial-measure beat positions — not as if the pickup were beat 1 of a full bar.

A one-eighth-note pickup followed by a 4/4 bar of eighths gets its beat-group cycle counted from the pickup note as if it were beat 1, offsetting every subsequent beam boundary in the first full bar by one eighth note (0.5 beat).

Gould, Behind Bars, ch. Beaming, discusses pickup/anacrusis beaming as a special case requiring the engraver to align grouping to the implied downbeat, not to note-count-from-zero.

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