it took some time until I was able to replicate your "issue" and I think I succeeded:
Well, this is not a bug but the attempt of the renderer to make things match. The growing textfield content is not of the same "height" as one of your internal, right side row elements. It calculates the remaining space until the page end and will fill a gap with empty if the remaining size is less than the height of your TextField row. Because you have lines all around you can see the gap. You would not get it if you would not have a dynamically growing right column row.
You can even see this in the design when you adapt the previous objects in height
Since this is a conflict situation for the renderer I don't think you would get rid of it if the general setup of your layout is as shown.
You could obfuscate the page break by removin the lines of your internal textfield row - but that is just hiding the obvious