Campbell, I believe it is one of these commits that caused the issue (I think the more recent of the two, but can't recall for certain.)
Jul 26 2017
Apr 3 2017
Nope I haven't improved the design, and I agree that that would be a great improvement.
I agree that it should be the default behaviour, eventually I'll make the old behaviour the option (screen spacing). Not quite ready to do that yet because I don't think it always gives better results.
line and curve are both 'space' strokes, and thus should have the space options available.
Bug fix, I was doing an clamping and scaling that was unneccessary resulting in any brush that wasn't close to 50px radius getting the wrong spacing, sometimes dramatically.
Mar 31 2017
Found the bug(s) wasn't calculating adaptive spacing so was always doing 1% doh! and then I was using the view_normal instead of true_view_normal. All seems to be working right now.
Mar 18 2017
It works fine here in the current released version of Blender, is this still an issue?
Poke - browsing old bugs is this still an issue?
Mar 15 2017
Added versioning so that sampling_range for area_normal is added to brushes.
new version of patch - this includes the 'sampling range' for planes. Will split into two patches soon. Sampling range allows the radius for determining the area plane to be smaller/larger than the radius used for sculpting the geometry - this allows the user to more precisely determine which faces they want to use to determine the plane when using a locked plane, or make the brush follow the surface more/less closely when using an unlocked plane.
Mar 14 2017
Mar 13 2017
Mar 8 2017
Closing since patches are preferred to be handled in 'differential'. Moved both patch versions to differential.
The updated patch now is part of the C function instead of the python function and the user can now optionally have the shading and non renderable objects disabled as per Campbell's suggestions.
Here is the revised patch, with the changes you suggested.