Can you provide an example of what you mean? The two lines don't overlap, do they? It may help to see the actual code in case there is another problem with it.
Also, if this turns out to be a bug with App Lab, we can let the student know. Otherwise, please be reminded that the Create guidelines state teachers may not "write, revise, amend, or correct student work, including debugging the program, writing or designing functionality of the program, testing the program, or making revisions to the program".
If possible, share the code or reproduce the problem on your own and share that code. I can try to see what's going on. Thanks!