Consider the following scenario. In Microsoft Word, you combine form fields (Legacy Tools) and content controls in the same document. Then, you enable the Filling in forms editing restriction. In this scenario, you cannot use the TAB key to move freely between the form fields and the content controls. You can tab from a form field to a content control. However, you cannot tab from a content control to a form field. Therefore, when you reach the first content control, you can only move between the content controls.
This issue occurs because the form fields and the content controls cannot work together.
To work around this issue, use content controls only.
This behavior is by design.