I'm on Affinity Designer v1.10.4
Vector brush strokes look too thin after using 'Expand Stroke'. After some experimentation, it appears that this happens only when Size Variance is not at 100%.
Scribble a new vector brush stroke
Select the move tool
In the 'Stroke' panel click 'Properties...'
Set Size Variance to 50%
Set the controller to Pressure
Set the controller profile to any of the standard profiles
Close the Properties
Set the pressure of the stroke to something non-trivial
Click Layer > Expand Stroke
Expected: The stroke will be expanded without changing width
Actual: The stroke becomes thinner
It manages to take the controller profile into account okay, as long as the size variance is at 100% so one workaround is to set the size variance to 100% and then adjust the controller profile to have the same effect.
This also affects PDF export.
expand_stroke_size_variance.afdesign
expand_stroke_size_variance_demo.mov
expand_stroke_size_variance_workaround.mov