Have you made the Main page flowable and allowed pagebreak in all flowable subforms and on the main page?
If it still doesn't work, share your form. I'll look into it.
Yes, that's the case with the positioned layout. You can do one thing if you do not want to change every subform to a flow layout, then let it be positioned and wrap them under a subform with a flowable layout.
Yes, it will jump because you have made others as positioned. So make other flow as well. It will be good for future changes, if you need to add/delete any fields in the future then the flow layout is best, it adjusts the form itself.
Use the flow layout for the Main page and for the subform if any are used in the form.
Also, make the field expand to fit and make it multiline from the field property.