I was recently producing a set of icons and encountered an annoying error when expanding shapes with rounded corners. I've previously posted this on the subreddit, which you can view here – https://www.reddit.com/r/Affinity/comments/6p9fjk/designer_expanded_strokes_are_not_straight/
Essentially when you work at smaller scales and use a border radius of less than 25% (I was using 15% I think) the edges are not perfectly straight as you'd expect. This is more obvious the lower the radius value and the smaller the stroke width.
Before expanding – http://imgur.com/e6JStLV
After expanding – http://imgur.com/f8C2plU
To clarify this isn't a just visual bug, the actual vector data is affected, as though the anchor points have had their handles adjusted. I'm able to replicate this in version 1.5.5 by creating a rounded shape 35px wide with a stroke of 0.4pt, corner radius to 5 and using the expand stroke command.
Results – http://imgur.com/a/ttNuT
Worryingly. this still happens when you scale the shape up first. I tried it at 350px wide with the stroke scaled with the object and the curvature is still occurring though it isn't as obvious unless you actually look for it.
For what it's worth I'm using a Macbook Pro (Retina, 15-inch, Mid 2015) with OSX 10.11.6.