TL;DR: Have a PDF with Düsseldorf. Not found if searching for dorf.

1. Have a PDF with Düsseldorf in the text: Düsseldorf.pdf
2. Create a new empty library
3. Create a new entry: Use "Düsseldorf" in the title
4. Create another entry. Use "zzz" in the title
5. Attach the PDF to the zzz entry
6. Search for "Düsseldorf". Ensure that full text search is enabled
7. See that zzz is matched
   
8. Remove "D"
9. See that zzz is not matched
   

NOTE: It might be necessary to close and open the library to have the search working

@Article{,
  title = {Düsseldorf},
}

@Article{,
  title = {zzz},
  file  = {:Duesseldorf:PDF},
}

@Comment{jabref-meta: databaseType:bibtex;}

@Comment{jabref-meta: fileDirectory:.;}