There's a reason I ask for those files. Didn't think I'd need to justify the reasons but if you want all the gory details, here goes...
A full screenshot showing the setup just before you print shows me the live view, how the print(s) are laid out, what placement method you are using, what size you are printing, the page size you chose, whether crop is on or off, and
many other parameters. It helps me recreate the same conditions for testing. A screenshot or file (since you are printing to file) showing the actual "line" you are referring to also shows me what you are talking about. There are a lot of things that can be described as a "line". It could be a solid line, broken line, noise, part of a shadow box, ghost window, and there are many of those that I can recognize right away (none related to Qimage) but I can't if I cant see what you are doing.
The original file is needed because often the file is corrupted, has some sort of incompatible transparent or alpha mask, or some other issue. I have Affinity Photo and while I have not had any issues, I have gotten sample files from others where I have found that it inserts a corrupted transparent background in some cases. Again, I need the original to see. Keep in mind that no one else is having a problem so obviously I need to see your files since I have no other way to see what is happening.
I already know that Qimage cannot create lines on its own so the line is either in the original (either there visually or is the result of some file corruption) or a parameter is being misused (the screenshot typically shows me that if it is the problem). Qimage also cannot, under any circumstances, corrupt a file. That has never happened in the 25+ years of Qimage. It simply can't happen because even when you edit an image in Qimage, it is non-destructive: Qimage has no code in it anywhere that can overwrite or alter originals. So I already know Qimage did not corrupt or alter your files.
My email is the same as you find on the support page if Qimage:
mchaney@ddisoftware.comMike