Hello, I created an account just to comment on this problem. I was blown away with Affinity Designer after discovering it recently -- it is the first serious contender to Illustrator that I have come across, and is a lot better than Illustrator in many ways. That excitement led to extreme disappointment when I ran into this problem and a bit of googling revealed that it is a known, longstanding issue.
Here is another example using the AMAZING new isometric tools, exported from Affinity Designer as a PNG:
Here is the same group of objects after importing the file into Illustrator and then exporting it as a PNG:
So I guess the workaround is create an image in Affinity Designer, export it to Illustrator, and then use their PNG exporter to fix Affinity's mistakes?
This is a very sad problem for such an awesome vector program to have. Your users shouldn't have to add outlines, make clones of images, or suffer through any other convoluted workarounds to mitigate a problem that clearly other programs figured out a long time ago. The whole point of vector illustration is to empower artists to create clean and elegant art; having to add in unwanted outlines or extra clones of objects to hide these hairlines defeats the purpose. If this was a problem the developer was actively working on, then great, and apologies for harping on the issue. But obviously that's not the case given their past comments and how long it's been an issue. I hope the team actually treats this like a top priority and figures out a fix soon. It really is one of the only issues holding it back from being a mainstream alternative to Illustrator for everyone who is sick of Adobe's crap.