To reproduce this bug:

1. Open the attached project
2. Open the ActivateInputFieldTest.unity scene.
3. Press Play.
4. Click on the input field and type something, then press Enter to submit
5. Notice how the input field onSubmit event does trigger the ActivateInputField action, but fails to actually reactivate the input field

InputField.ActivateInputField() has no effect when invoked from the input field's onSubmit callbacks.

Workaround:

Delay input field reactivation a few ms with Invoke()