I think I've had a similar problem but I'm a complete newbie not just to Affinity Designer but to just using any graphic/image manipulation apps as well as mac and it feels a bit too much sometimes-it's only been a month. I tried to learn Gimp last year but it's such a mammoth beast I just drowned in it and couldn't make head or tail of what was what. AD seemed like a fair priced more lightweight risk to take and on the whole I'm happy with and actually getting to grips with it-I think I'll be able to do what I'm trying to do re: book covers/interiors for KDP.
Sorry I'll stop blethering and get down to it. I've combined individual pdfs for each page into one pdf to make a book interior and using the preview on mac everything looks exactly the way i want it to, images, layouts and fonts. However when I open the new pdf manuscript in AD with all the pages on one artboard the fonts are changed even though the two fonts used throughout the document are on the computer-I use them in Libre-write all the time.
Does preview on the mac accurately display what will be uploaded to KDP or anywhere else? Or is the AD version of reality the real one and I have a problem? If there's a problem and it has a solution, please put any explanations in simple terms.
Yours, a bemused former Luddite from a washed out Kodak snap from the 1970's. That yellow vinyl sofa was never so pale in real life...