It seems as if you just found one of the features that differ between
version 2.8 and 2.9. I don't know any details but it's probably related to
Erik's major revision of the front end of LilyPond.
One simple solution that works well in both 2.8 and 2.9 is:
fingerscript = {
\once \override TextScript #'font-size = #-5
\once \override TextScript #'font-encoding = #'fetaNumber
}
\relative
{
\fingerscript c1-"1-2" | c | c |
}
Unfortunately, this low-tech solution doesn't use your newly acquired
skills
in writing music functions. ;-)
/Mats