For any who are struggling with this:
The solutions posted here didn't work for me. I added bleed in Document Setup, then turned on Include Bleed in my PDF exports—no bleed. It makes space for bleed, but does not give me bleed. To test/prove this, I set bleed to 3" on an 8.5x11 document and made a bunch of green rectangles in the bleed area—see attached screenshot of the interface, and the resulting pdf.
No combination of setting/unsetting/resetting bleed in document setup or export options fixed this, and switching "clip to canvas" on/off had no effect either. The only thing that allowed it to work was if I turned off my live effect layer—then, suddenly, bleed on export worked as it should (though the images looked not at all as intended). So I grouped the live effect along with the images, rasterized the group, and it continued to export bleed properly. To be clear, this is a workaround—the fact that a workaround exists doesn't mean it's not bugged. It means it is bugged.
"But, dude," you say, "live adjustments are an Affinity Photo thing, not an Affinity Designer thing, so why would you expect them to work in Designer?" Well, if they're going to tout as a selling point that the programs are interoperable on the same document, those programs need to actually support each other's features. Designer displays the live layers just fine, so it's bizarre that it can't extend them out to the bleed.
blankbleedBig.pdf
blankbleedBig.pdf