I noticed when executing "Expand stroke" on a smooth curve that the resulting vector has one outside node that's sharp on the left while the right is smooth. This seems to happen when rotated certain ways but other rotations result in it being all smooth across the entire outside. Also noticed that if I make the sharp node smooth the node at the top (connected on the left side) jumps down.
Screenshot added. Example file attached.
Let me know if I can provide any other information.
Versions
macOS: Big Sur 11.5.2
Affinity Designer: 1.10.4
curved_line.afdesign