Melisma extender line is drawn only for a genuine multi-note melisma and stops exactly at the last note it covers

When one syllable is sung across several notes with no new syllable arriving, a plain horizontal extender line is drawn from just after the syllable text to just before (or under) the last note of the melisma — never shorter or longer than the actual span.

A melisma covering notes 4-7 has its extender line rendered only to note 6, or the line bleeds into note 8's column where a new syllable begins.

Gould (extending a syllable over several notes); Read; MusicXML extend type start/stop.

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