Thank you for the detailed explanation.
I still don’t see why this is not an issue. As a designer, you can’t always stay pure vector. A lot of times you have to deal with printing. And this can cause some serious issue, especially when you have multiple art boards. I didn’t run into any similar problems on Desktop versions of Illustrator and InDesign. If Affinity Designer sees itself as a professional software, they should “solve” it as I mentioned earlier.
When the document unit is set, everything should be sticking to that unit, and maybe try it’s very best to convert to that unit. In this case, millimeters.
This is just my personal opinion, I might still wrong, but what I want to do is having a thorough discussion on this.