LR 13.5 incorrectly changed the TranslatedStrings keyboard shortcuts for Secondary Display > Loupe - Live to use invalid syntax for shortcut definitions. The shortcuts, defined by these ZString keys:
$$$/MultiMonitor/LiveLoupeShortcut/Win
$$$/MultiMonitor/LiveLoupeShortcut/Mac
should be "Shift+Z" (no spaces). But instead they have these values:
Note that the keyboard shortcut for Loupe - Live doesn't work on Mac but it does on Windows:
https://community.adobe.com/t5/lightroom-classic-bugs/p-shortcut-shift-z-loupe-live-doesn-t-work-on-...