This article (in French) gives another possible reason:
the independence of the European Commission has increased under Jean-Claude Juncker's presidency and this does not please big European countries like France and Germany. The secretary-general of the Commission, Martin Selmayr, was close to Juncker and instrumental in that independence.
Ursula von der Leyen will soon replace Juncker at the presidency of the Commission but this comes with a condition from France and a few other European countries: Selmayr should leave because these countries are hostiles to the presence of two Germans at the head of the institution. France is now trying to obtain the position of secretary-general of the European Commission.
The problem is that many countries, especially in Eastern Europe, argues that the position should fall to a country that less well-endowed in terms of important European positions. Supporting the Romanian candidate for the position Chief Prosecutor of the European Public Prosecutor's Office would be a way to please these countries and look less greedy.
Another possibility, coming from French politics but more speculative. This other French article explains that, in its short life, the Parquet National Financier (PNF, the French judicial institution responsible for tracking down economic and financial crime) has already shown to be independent from the executive power and has some major exploits to its credits (the case of the then-presidential candidate Francois Fillon and the investigation on the bank UBS which lead to a 3.7 billions fine).
Now, the PNF will have to handle the cases of two individuals known to be very close to the president Emmanuel Macron: Alexandre Benalla, who is accused of financial relations with two Russian oligarchs during his time at the service of Emmanuel Macron, and Alexis Kohler, Chief of Staff of the Presidency of the French Republic, who is under investigation for corruption.
Nominating Francois Bonhert at the head of the PNF, far from a consolation prize, could be a move from Emmanuel Macron to help his friends and regain influence on the institution.