Argument ByRef in a Function when not using the return value.

I have noticed an oddity when passing argument ByRef in a function when not using the return value. Take this example:

Option Explicit

Function Triple(ByRef x As Integer) As Integer

x = x * 3
Triple = x

End Function


Sub test()
    Dim x As Integer
    Dim y As Integer
    x = 5
    y = Triple(x)
    Debug.Print x
End Sub

It print 15, as expected. Now remove the assignement when calling Triple:

Option Explicit

Function Triple(ByRef x As Integer) As Integer

x = x * 3
Triple = x

End Function


Sub test()
    Dim x As Integer
    Dim y As Integer
    x = 5
    Triple(x)
    Debug.Print x
End Sub

To my surprise, it prints 5. Why this behavior? Is it documented somewhere? (I am on Excel 2016).