I have been suddenly having this problem appear when I try to use any of my custom brushes in curves. The brushes appear correct when done in straight lines, but the fill appears anytime I try to use them in curves.
With the brushed path selected, make sure there is no fill shown. A brushed path can also have a fill. The fill could also be added to the layer as well.
Before applying Brush to Curved Shape:
Click on the "/" key on the keyboard - which will make the Fill None.
Before creating the brush, make sure that there is no fill applied to that line.
In order to edit the already existing brush, drag it out of the bruhes panel to the artboard. Then you can just take the direct selection tool to select that line and then turn off the fill.
Thank you so much! This had been driving me crazy!
I can imagine that. Glad you could solve it now.