@lacerto Thank you for your detailed explanations. I've been pulling my hair out trying to figure out if my issue was intentional/standard or a bug. I didn't remember previous print projects looking so drastically different in PDF export (CMYK from the get-go, export as CMYK PDF) but I guess I was just using colors that were cross compatible in the past (whatever the proper terminology is there haha).
I have my business card project started in CMYK using US Coated SWOP v2, exported with PDF for Print using Document Color, all that jazz, and the preview window is WAYYYY off. It's not actually stated anywhere that I could find online—be it Affinity docs, guides, random Youtube vids, posts on Reddit and on Affinity Forum—that the Export Preview window is supposed to be off. I feel like I have a superpower for being able to find myself in weird edge cases, esoteric bugs/quirks, or just between-the-lines presumed knowledge that's never stated with any software, OS, or tool I use. It's maddening.
Not trying to redirect or resurrect someone else's thread, just wanted to say thanks because I was in the same situation.