maybe it is not completely a bug, but I believe this should be fixed if it is a software fault.
Problem: If I use Layer > expand stroke on rounded curves results are not as expected and what is interesting - this effect intensifies when the curve has a smaller size. The picture below shows 12px wide circle affected by what I am talking about.
I figured out that I can draw a large circle, expand it and scale it down for better results - but it's a poor solution for the long run.
My Affinity Designer version: 18.104.22.168