To reproduce:
1) Create new project
2) Create C# script with this in it:

string foo = "0";
void Update() {
if (Input.GetKeyUp(KeyCode.W)) {
Debug.Log("Button is up");
foo += "1";
}
}
void OnGUI() { GUI.Label(new Rect(10, 10, 1000, 200), foo); }

3) Add it to any object in the scene
4) Play the scene
5) Hold 'W' key
Expected: Nothing is logged until you release 'W' key
6) Build and run on Linux player
7) Hold 'W' key there
Actual: Now the if is constantly true while you are holding the button

Reproduced in 2017.3.0f1 and 2018.1.0b8 Linux editors

Not reproduced in Windows Player