here is the view from a 12.9" latest gen ipad pro.  i'd guess what happened is in your template the background with the lines is just obeying the laws of physics and trying to scale for mobile..

i would suggest not using something with tightly spaced lines like that for the reason that it's not a SVG image, its not going to scale correctly to different display sizes.  and it will look very strange depending on how you saved the file.  SVG files can adapt dynamically to different display sizes, but i don't know offhand if the una script natively supports this, and instructs to render per viewport resolution change from mobile or desktop, etc.
you bring up a good thing for people to be mindful of.