In 9.1.6 Multiple syllables to one note
<https://lilypond.org/doc/v2.27/Documentation/notation/common-notation-for-vocal-music#multiple-syllables-to-one-note>,
the apostrophes within lyrics should be curly (’) instead of straight (').
Here is a revised example:
{
\relative {
\autoBeamOff
r8 b' c fis, fis c' b e,
}
\addlyrics
{
% Ensure hyphens are visible
\override LyricHyphen.minimum-distance = 1.0
Che_in ques -- ta_e_in quel -- *l’al* -- *tr’on* -- da
}
\addlyrics { "Che in" ques -- "ta e in" quel -- *l’al* -- *tr’on* -- da }
\addlyrics { Che~in ques -- ta~e~in quel -- *l’al* -- *tr’on* -- da }
}