I found a nasty little bug when working with constraints groups that only happens on duplicated art boards.
Here's how to reproduce:
Create a new document in Affinity Designer with a single artboard (e.g. for "Devices").
In the "Layers" panel, right click "Artboard1" and hit "Duplicate".
The new artboard appears on top of the old one. Move it to the right of the original one.
In the artboard on the right, create any two shapes (e.g. two boxes).
For one of the two shapes, enable "Min Fit" in the Contraints panel.
Now select both shapes and combine them into a single constrained group via "Layer - Group".
Notice how the shapes disappear, and a new bounding box appears at the right of the current artboard. Oops!
Thanks in advance for fixing this — and keep on the great work with the Affinity products!
[Reproduced on MacOS 10.14.6 using Affinity Designer 1.7.2 from the Mac App Store.]