In a game with return to placement enabled, I noticed that if you place units and switch to Attack without Accepting the unit placement, the unit count remains visible on the board.  However if you try attack from the territory with the phantom units, you cannot use them.  The territory unit count displayed is only updated if you return to placement, place additional units on the territory and then Accept.

This has been brought up before and is on the to do list as far as I remember.

Thanks WW I'll update - had lost track of this one.