Indeed it often doesn't show that AP is subtracted. Or there is often substantial delay - not only from when an order was given but also after the action was already done.
However, oddly seems that it probably always correctly updates the AP indicator when you switch the weapon mode by left click. So I've gotten used to left clicking when taking an action to see how many APs are left correctly.
I don't know whether it helps anything, but base on this experience I would wild guessingly recommend to use this function that is called during weapon mode switching (for the AP status display update) also at all other instances when we actually do AP costly stuff. Of course it needs to be called right away at the time an order is given. Though in case you would want to have some move interrupts in future, I can imagine the AP subtraction shown gradually during movement action.