@Doitsu
Thank you for your reply. Today I learnt something.
It's indeed a commercial font.
I included ‍ in the ligatures to see what happens:
- Sigil is impressed and displays a nice ligature.
- Kobo is not and still leaves a blank...
- Prince stops whining and now accepts to print the file. However, it would be very OK if I did not see this:
Same result using: &-#8205;