Ah, j' ai finalement trouvée ma réponse sur le support de Rockstar :
After passing into legend, my Outlaws to the End co-op challenges are not advancing and co-op achievements are not unlocking after completing the requirements. What can I do?
Answer
We are aware that co-op challenges/achievements from Outlaws to the End are not updating for some players after passing into legend when partial completion was resistered before the prestige switch. This issue affects co-op/class-based challenges as well as achievements/trophies such as "The Mother Lode" and "You Rule!", which are based off of co-op progress and performance.
The fix for this issue is will be deployed in a Title Update near the release of the Undead Nightmare DLC pack.
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en gros, y' a des bugs au niveau des défis en coop pour ceux qui ont passé un grade de légende, peu importe lequel. Et un patch (un de plus...) sera déployé plus ou moins en meme temps que le prochain DLC "Nightmare"
