Minimize/Maximize/Close decorations disappear when title bar is turned off on the customize screen

Running Manjaro Linux with KDE 5.19.5, Kernel version 5.8

Steps to reproduce

1: Open Firefox

2: Go to the 'Customize' screen (Hamberger Menu -> Customize)

3: Uncheck 'Title Bar'

4: The buttons may appear briefly after the window repaints, then disappear after a few seconds of interacting with the app.

I have observed the same behavior in firefox-ubuntu from the AUR. However the Firefox build in the official Arch repositories has the correct behavior, leading me to believe the bug is related to the patch that enables global menu support.