With a function layer activated with ACTION_LAYER_MOMENTARY bound to a fn key, if the fn key is released before the modifier key is (physically) released, the modifier key is never marked as released, and continue to be registered as pressed to the computer.

This bug does not occur if the key is a normal key (tested with left direction key).

To reproduce:

1. Bind a layer with ACTION_LAYER_MOMENTARY to a fn key.
2. Map a key on that layer to a modifier key (tested with RSHIFT, RCTRL, RALT, RGUI)
3. Press (hold) fn key + modifier key
4. Release fn key and then release modifier key
5. Observe that the modifier key is still registered as pressed

Version: git master

Board: gh60 (rev. chn) (only difference from rev a/b is different gpio pins for matrix)

Config:
lufa build
BOOTMAGIC_ENABLE = yes
CONSOLE_ENABLE = yes
COMMAND_ENABLE = yes
USB_6KRO_ENABLE = yes
BACKLIGHT_ENABLE = yes