Figures align horizontally with the bass note's onset, not with a generic beat grid

Each figure column sits directly beneath the bass notehead it describes — if the bass moves (even a passing tone), the figure column moves with it; figures never float at a generic beat position disconnected from where the bass note actually falls.

A figure entry's rendered x-position drifts from its bass note's x-position, e.g. because the renderer places figures on a fixed per-beat grid rather than reading the actual (possibly syncopated or tied) note's x-coordinate.

Gould; Read (figures placed directly below the note to which they refer).

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.