I have never had this issue before.
I imported a .png file and then using another layer traced the outline. Normally the tracing is on top of the .png file but now my lines all go back behind the .png layer. I can easily select the .png layer and "Send to the Back" which solves the problem for lines already drawn. However, as soon as I try and draw a new line on the other layer again the line is automatically sent to the back behind the .png image.