Here it is.
This is what I did: I wanted to provide a real book and not a two page test book. So, I selected one of the EPUB2 I produced this year (in French), I inserted the words or expressions of the
petiteliste.txt without taking any care for meaning and saved. So I apologize for those who wish to read it, they will face some understanding problems...
Then, opening Sigil 9.2 (Archlinux x64 build), I inserted the text file in the window of the index editor, saved and close it. Then I generated the index.
Result: the same phenomenon occurred: this time I had two entries (instead of three) in the beginning before the a, and the French diacritic was placed at the end, so the two main problems are confirmed with this Sigil build.