I’ve just been experimenting with your two examples. I surmise the following:
Dorico can automatically adjust existing extender lines, and generally does so remarkably well.
BUT
When a lyric is initially entered, Dorico decides whether an extender line is needed or not. If the note value changes, Dorico doesn’t recalculate this. If the lyric started off with an extender line then (unless you re-enter the lyric) Dorico will keep the extender line. If the lyric started off without an extender line then (unless you re-enter the lyric) Dorico won’t add an extender line.