When bringing in multi line textboxes from any source I should be able to choose a textbox control with a scroll bar within so that my field does not get cut off.
In the meantime, you can use a blank, flexible height Gallery with an auto-height textbox.
Set the Gallery source and filter based on the record selected by the other instance of the source. Then size the Gallery to the space you would want your textbox to occupy at its largest. When the auto-height textbox would exceed the space of the Gallery, it will have a scrollbar.
This is not scalable, but for one or two textboxes on a screen, it achieves its purpose..