How does Rain of Arrows/Radahn's Rain work?

[Wrathios]

People invading are somehow casting this AoW onto the host without line of sight and behind a wall instead.

I've tried this but the lock on breaks and the AoW just happens around me instead.

I'll run away or get behind cover but It seems like no matter what the host or I do, we're getting caught in the AoW. I do not understand how these invaders are spamming this AoW with much success.

[Dollmaker]

You can lock on if you can see the target, even if they themselves cant, it will potentially break shortly after, you gotta cast the AoW before it happens.

[Imperator Ricardo]

There is a small grace period where lock-on will be retained even when visual contact is lost, they simply time their weapon art usage to be within this grace period
As long as the arrow is being fired while lock-on is still there, the target will get hit, even if it disconnects right afterwards.

A minor addition is that most Invaders use Golem Greatarrows, wich have splash damage, though it is much less impactful than the lock-on, since splash damage does not help at all if the arrows just land right back on the archer that fired them.