After changing any settings in the Window Management section, kwin disappears from Global Shortcuts until kwin is restarted.
I was wrong... kwin doesn't disappear from the Global Shortcuts list; it gets renamed to 'System Settings'. Just reinstalled Arch and the bug is still present. I can only reliably reproduce the bug by changing the settings in Window Management -> Task Switcher, but other subsections in Window Management still trigger the bug on occasion.
*** This bug has been marked as a duplicate of bug 395599 ***