It's not a problem or a bug, it's obviously made intentionally, because the way of KLAX (and KDFW) ground is made, airplane shadows wouldn't be visible anyway after a certain altitude so, it doesn't make any sense to keep them on, killing fps, when you couldn't see them in any case.
But, this is happening ONLY in the airport range, when you exit from the airport range, the value will be set back to what you had before.
However, if you quit a flight or exit from FSX for any reason while in the airport range with shadows turned off, FSX will *save* this setting in the FSX.CFG, so there wouldn't be any other way for us to know if the setting was set by you intentionally, or was saved by FSX because you quit when the shadows were turned off automatically by KLAX and you were in the airport range (which is from 10 to 16 NM at most).
So, don't quit if you are *flying* and in the airport range, if you need to quit from FSX inside the airport range, do it when you are on ground.