To reproduce:
1. Open the attached project
2. Open the ActivateInputFieldTest.unity scene. Notice how the input field is scripted to invoke InputField.ActivateInputField() from OnEnable()
3. Press play. Notice how the input field does NOT get activated by the script even though it did invoke InputField.ActivateInputField()

InputField.ActivateInputField() has no effect when invoked during script initialization from Start, Awake or OnEnable.