Just a question i have but is it just me or is Mu's Fission clone Y not triggering correctly or is it merely a badly worded?

From what I have seen so far and subtly tested, it seems that it really only triggers it's own mystery when the opponent unit has non-ninja units (clones, puppets, summons) in their team by the time it is summoned. For example:
If Breeze Dancer has clones on the field already before the clone is summoned, the mystery is triggered.

Not only that, the mystery by the clone ONLY seems to damage non-ninja units. Every other unit will be ignored and dmg is only done to non ninja units as a result.

Is this a bug or intentional?