Em dash missing from WM_NAME with libx11-1.8.2

Firefox's window title generally looks like "Web Page Title — Mozilla Firefox", but after upgrading libx11 from 1.8.1 to 1.8.2, it now looks like "Web Page Title Mozilla Firefox". The em dash that was previously in the title is somehow getting dropped.

Reproduced on Arch Linux with XMonad 0.17.1.
Attached is a video showing the problem.
simplescreenrecorder-2022-11-16_22.03.25.mkv