I'm not sure if I am missing something, but if I run the parser on the string "de Mesnil", I am expecting it to give me either a first or a last name of "de Mesnil" (preferably the latter), given that "de" is a known prefix.

Instead I am getting a first name "de" and a last name "Mesnil".

That seems contradictory to the documentation for prefixes: Name pieces that appear before a last name. Prefixes join to the piece that follows them to make one new piece.