Pressing the Fn key triggers unbound keybinds and interrupts held movement keys

For example, if Cinematic/Smooth Camera is unbound, pressing Fn activates it. If I assign Cinematic/Smooth Camera to a normal key such as F8, pressing Fn no longer activates it.

This happens in completely vanilla Minecraft.

Expected: Pressing Fn shouldn't do anything beside allowing us to press stuff like F1 or F4.

Actual: Pressing Fn can trigger unbound actions and cancel held movement input.