I just Got 2 Girls with the x10 Drop girl (2 Koban) , one regular girl from campaign and another one from the current event (Retro-Future Girls), but only the regular appear as a drop on the loot screen, and I just see event girl appear in my harem a few second later.
It make sense when it's "TEN Times ONE Drop" Drop for gaining time , and not the 900 Koban drop where it's "ONE Time TEN Drop" ... Just a matter of luck .