Ok,i just run a test in the editor to see if i'm right or not.Of course,since my test involves only 5 spotters it must be taken with care,still what i've seen so far seems to prove my point.
Here's how it goes:5 german 120mm spotters have been aligned behind a hill.In turn 2(to avoid pre-plotting),all 5 receive orders to target
without having los to the target area.
They are also instructed to run forward(with a delay of 14 seconds) in order to gain los to the target when the spotting round will land.
As you can see from the below image,they now have los to the target area
before the spotting round was fired.
The spotting rounds land.4 of them are in sight of the FOs.
...one however,isn't.
And the end result:
So as you can see-and please feel free to test it yourself-all the rounds landed on-target although in the moment when the arty strike was actually plotted neither of them had los to the target,nor did one spotting round was seen when it felt.