It has been a Bug since before version 9, I came across it reported by Werner, @WSNetwork, in 2011.

It appears to be connected to tapering strokes.
The 'trick' is to explicitly or implicitly force the lines to render as images.
Explicit - add an Image Filename and set as PNG - tedious.
Implicit - add the smallest possible Feather of 1mp and you force an image for anything (except Placeholders) - dangerous, but useful.

Acorn