While Goto Definition works as expected with key F12,
the hotkey Alt+F12 for Peek Definition no longer does anything.
How do I restore Alt+F12 functionality?
One cause of this is Nvidia GeForce Experience intercepting Alt presses while listening for Alt+Z, which is the activation toggle for Nvidia ShadowPlay.
Fix this by opening GeForce Experience and going to Settings -> General -> In-Game Overlay -> Disabled.
(Thanks to @Levi Fuller for the updated solution)