Is the line showing up when published? I’m just asking because bezier curve lines do not get shown on publish and this might be a similar scenario. If not then you could just ignore it. If it is showing up then it obviously needs fixing. It’s most likely a positioning issue as you’ve hinted at.
To track it down requires some elimination. Can you get the line to hide if you make set the mapped plane to not be visible, or perhaps some other element? Is it always visible?
If you zoom in on the line does it have a colour? If so, can you change the colour? Can you select the line at all, this would show on the left which element it’s attached to.
just some ideas… good luck