\documentclass{article}
\usepackage{xhfill}
\begin{document}
Foo \xrfill[0pt]{30pt}[black]
\end{document}
When rendered by LuaLaTeX, this produces a thick black line with clearly visible vertical stripes:
When rendered by XeLaTeX, the line is solid as expected.
This seemed to come and go at first. I went down several false paths, including the % !TEX program
string, newlines, LaTeX Workshop vs TexShop, and clearing aux files.
Edit: It may be viewer-related. I can see lines in Preview.app and Tex Shop (which may use an embedded Preview pane), but not in Chrome. When viewing with Preview, I can consistently toggle stripes/no stripes by building with XeLaTex/LuaLaTex.
Environment: macOS Big Sur 11.2, MacTeX-2020, TeXShop 4.58, all packages up-to-date via TeX Live.