Why Key Test app shows KEYCODE_UNKNOWN when I press headset control button when the Generic.kl contains KEY 226 HEADSETHOOK?

My Phone runs Resurrection Remix 7.1(Redmi 3s). When I press headset control button, the key test app identifies the code as 226 but says KEYCODE_UNKNOWN. But the Generic.kl file contains the line KEY 226 HEADSETHOOK. Also I cannot play/pause the music using headset control button.