F5 key not working consistently

I have a 2012 Macbook Air 13" notebook. It was working great until this week. The F5 key that dims the keyboard backlight is not working properly like before.

Scenario 1: when the Macbook Air is in a low light environment, the F5 key works properly -- meaning each time the key is pressed, the backlight gets dimmer by 1 bar.

Scenario 2: when the Macbook Air is in complete darkness with no lights on, the F5 key would be slow in its response -- meaning for every 2 times the key is pressed, the backlight gets dimmer by 1 bar.

I've tested scenario 2 in day time also by covering up the camera/ambient light sensor completely with thick paper. The results are the same as in Scenario 1 and 2.

I've already taken this problem to Apple and they have already replaced the top casing with keyboard. THE PROBLEM STILL EXISTS.

Question: Is Scenario 2 normal? It's no big deal because I can still dim the backlight by pressing and holding the F5 key until desired level. However, I like to know if it's not working like it should be in Scenario 1 even if Macbook Air is in complete darkness.

Thanks