I am fairly confident that a lighthouse is not supposed to be shining light onto land, but the lighthouse in Arma 2 and Arma 3 does so, and as such I wonder if this is unrealistic and if it is, I hope it can be fixed.

Perhaps lighthouses in the Eastern Europe work differently?

I would expect a lighthouse to shine light down 'sectors', so that ships in a green sector have a clear path towards the lighthouse. An area towards land would be a sector where light does not shine. I am not 100% sure but I am guessing that simple shields inside the lighthouse are allowing the rotating light to shine out to the sea but not towards land.

Another aspect here could be that a lighthouse does indeed shine in a 360 arc when on an island, but then perhaps the light should not shine downwards onto ground (and upwards into the sky).