Affinity Designer version: 1.7.2.471
OS: Windows 10
Reproducible: Yes, but not with a new document.
Steps: Select artboard(s) to copy, select other document tab, paste artboard(s)
Video attached shows the issue, I copy and paste a number of artboards to a different document, and the strokes of objects (thousands I'd guess) changed. I checked whether it had something to do with the current stroke width in the second document, but setting it to 0 produces the same effect. However, while pasting these artboards to a new document produces the same effect, the stroke widths end up different. When I paste the artboards to a new document, the widths increased from 0.5pt to 1pt, in the video below they are 4.2pt. Though that also seems to vary depending on the object.
Affinity Designer 2019-09-17 16-51-00.mp4