A tie whose two notes straddle a barline is normal and must not be flagged

Ties routinely and correctly cross barlines (e.g. a half note tied into the next measure's quarter note) — this is one of the primary reasons ties exist.

A naive 'tie should not cross a barline' rule would be WRONG and would false-positive on completely correct notation — noting this explicitly since an engineer implementing tie checks from first principles might over-apply barline logic by analogy with beam-grouping rules (beams DO break at barlines; ties explicitly do not).

Gould devotes significant space to bar-crossing ties as the standard, unremarkable case; Read same.

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