When entering play mode, the UI will be regenerated. Since the target TextField in the UXML file did not set a value, it will use the default one, which is an empty string.
When the binding is applied, it will set the value coming from the data source and if it is different, it will fire the value changed callback. This is all expected.
So although there might not be a visual change occurring, a change has indeed occurred.
As a side note, OnEnable/OnDisable should be prioritized to query elements and perform configuration. This is because the UIDocument will generate the UI on OnEnable and release it on OnDisable.