Event Pikmin are all unavailable to be sent on a mushroom after all mushrooms have been completed and rewards obtained. When selected in the Pikmin menu, they state that they are still busy with mushrooms.
Those Pikmin may still have the fruit from the mushroom reward, and cannot be sent to the next mushroom until you receive them.
I have cycled through the pikmin on my screen multiple times and no mushroom-completed pikmin or fruit-holding pikmin are ever present. If those pikmin are carrying items, they are not appearing.
That is why I think this is a bug. The pikmin never appear and cannot be interacted with in any way.