Thank you for the post.
Try the following:
1. Open the database with FileMaker Pro (Advanced).
2. Click “Edit Layout”
3. Click “Layouts” (menu at the top)
4. Click “Layout Setup…”
5. Uncheck the box “Delineate fields on current record only” (See screenshot below.)
Do the lines now show up in Preview or when printing?
Thank you very much for this easy but important info!!! That works just great :D