I own Affinity Designer and use it to create wedding stationery which I outsource to a local printshop for printing. I always convert the documents to a PDF before emailing to the printshop and have never had any problems before now. This is what is showing up on their end (on my end it is just a nice white border).
I have ungrouped every single element in the file and converted them all to curves. I feel like I've tried everything and I'm not sure what to do next to ensure it shows up correctly and can be printed for my customer!
* NOTE this is a screenshot that they sent me. I do my work on an iMac and it looks like they use Windows but like I said I've never had any issues in the past with them!*