Hello,
This text is unfortunately unclear, or lacking in additional relevant information. The passive mystery attack this clone does is only performed against non-ninja units, such as clones, puppets, and summons. This may be why you are seeing it trigger only once at the beginning of battle. The ability will not trigger if no non-ninja units are present.

In your experience, is your unit failing to execute the passive action even with such units on the field? Please let me know, thanks!

Hello,
I have submitted a request for the missing text to be updated, but it has not yet been adjusted.
Sorry about the inconvenience!