When I make a custom design, to be used as a brush and be applied to a stroke, it always seems to very glitchy when resizing or zooming in and out. This also happens when moving the object, or just moving the screen, as well as after the object has been moved and just sitting there.
I did a screen capture and posted it to YouTube so you can see what I'm talking about better. Also showing the same design in Illustrator.
This design was made in Designer and I used the same file to make a custom brush in Illustrator to do the same thing.
-Macbook Pro Retina 15 inch Mid 2015
-macOS Sierra 10.12.4
-Affinity Designer 1.5.5
-Adobe Illustrator 19.2.2