Hi! Assist mystery is based on your current "Assist links" that you have chosen. "Assist links" are specific to the lineup you currently have active, so switching that lineup will result in different "Assist links". I would recommend taking a look at your "Assist links" to make sure you meet the requirement for the Assist Mystery.

This is the same with Arena, your current lineup that determines your current "Assist links" determines if you met the requirement for the Assist Mystery in Arena.

You can find more information regarding Assist links here and here. Courtesy of ICE*