I'm quite surprised as, I did a bit of research and I've seen this issue has been going on since at least 2015.
I love the app and the potential it's got, but this a deal breaker. I don't think I can rely on exporting JPEGS from Pixelmator.
Judge for yourselves:
1. Affinity photo export
2. Pixelmator
3. Photoshop
I really wish the developers do something about this because this defies the purpose of any photo editing app. I remember a similar issue with Affinity apps from a few years back, I also complained about it and their v2 apps seem to have resolved this issue.