For me when I click a mystery near the end of the round it lags for example I use edo deidara, sailor sakura, jigokudo, and Earth main. When i used EM at the end of R1 when chakra is 40 it lags and the mystery is used 2nd round. Also, when i click a mystery lets say at the start of round 2 it goes off 3 secodns after I click.

EDIT: sometimes I click it before standard attack of that character and it waits until THE END of the round to pop.