This applies to Affinity Designer.
To reproduce this bug:
Draw on canvas (with Shape, Pen, or Pencil Tool) Choose Solid Line Style with any Width Modify Pressure graph Choose a brush from Brushes Panel (thus changing to Texture Line Style) Choose again Solid Line Style
After these steps, Pressure Graph isn't applied to Solid Line and if you try to modify the graph nothing happens. You need to Reset Pressure, choose again any brush, and than choose Solid Line Style again. Now everything works.
I can reproduce this bug ALMOST always, but sometimes (rarely) everything works fine, I couldn't figure it out why.