In case this helps, here is a simple way I can trigger the bug: I launch my web browser (iridium, which uses GTK3), go to duckduckgo.com, press Control+p to print, choose "Save as PDF", then click "Save".
The page name (DuckDuckGo – Privacy, simplified.) is automatically selected as the filename for the PDF and the dash (between DuckDuckGo and Privacy) triggers the bug. I cannot print to PDF until I delete the dash in the filename (replacing it with a hyphen, for example).