I'm going to GUESS that the white line you are talking about is a very fine space that is 'scuffed" from the sanding, and maybe the rubbing will not quite reach into the shallow spot to buff it out.
As far it going away if you re-cleared.....hmmmmm maybe, maybe not.
I would put a little water on the area and see if it goes nice and clear or it stays white.
If it goes away, and then returns when the water dries off, I would guess you could just re-clear it.
If the water does not make it go away......I'm not sure what to do with it.
Is it possible that the "white line" is dust or some other trash that built up around the edge of the decal, and then you cleared over it?
If that is the case......you are pretty much screwed.