Hi, it is highly recommended that you create subforms in your form. By creating parts in your form you should create subforms for each of those parts. For the textfield you are trying to make it expandable and flowable to the next page, you must make every of its parents flowed. Your main page is actually set to positioned, even if your subform containing the textfield is flowed, if each of its parents are set to flowed it should work, otherwise it will not flow to next page.
If you set your page to flowed, it will reposition each of your fields because it is not in subforms. Make sure to create subforms for each parts, putting all fields in subforms. If you set your main page to flowed without creating subforms, all of your fields will be redesigned to be aligned left with the hierarchy order.
Hope this help.