That is what I thought, too. But I tried disabling the "maximum difficulty" setting (which I had always on, I don't use/like Dynamic Difficulty), and going into riff repeater to learn a phrase, with level up on. Which did not succeed unlocking the mission, unfortunately... Plus, the game don't give everyone the same mission, and apart if the selection were completely random, it would not make much sense to give one based on dynamic difficulty to someone that disable it, wouldn't it ? But, damn. I cannot figure what else it could be.