Once fake player pick up some item, i can only get it back if it is on his mainhand.(KeepInventory is on)

Please consider adding a opinion "slot".

makes sense.

but... you can still do that right now:
/script run drop_item(player('<player_name>'), 3, 1) // drop one item from slot 3 of player <player_name>
/script run drop_item(player('<player_name>'), 3) // drop everything from slot 3 of player <player_name>

is that sufficient?

Thanks for your reply!
But, I am not operator and cannot run scripts.

I hope you could add that in the next verson.

Thanks you.

makes sense

added as additional options for drop and dropStack: all, mainhand, offhand or