Thank you for the post.
Since the field is a portal field, the only way to see the field on the layout again is to stretch the portal to encompass the field. I am attaching a screenshot below showing the portal stretched out to bring the field back into view. Not using the inspector to move the field and dragging instead also works. This is changed behavior from FileMaker Pro 11.
FileMaker has documented this change in layout widths in Knowledge Base article 10785:
In previous versions of FileMaker Pro, the width of a layout was defined by the right-most edge of the right most layout object on the layout itself. This width had no discernible effect on layouts that did not utilize auto-resizing and anchors. On layouts that did utilize auto-resizing, this width was used as the base width for the layout object auto-resizing calculations.
New to FileMaker Pro is the concept of a defined “right” margin for a layout. Previous versions of FileMaker Pro had an implied top, bottom, and left margin for layouts but the right margin was in a sense infinite. Therefore, if you wanted to define a right margin you needed to use a layout object, such a line, to do so.
Hmmm, so it's possible to place a field in a portal but then use the inspector reposition it so that, while still in the portal it is not visible due to being beyond the borders of the portal? Intriguing if so, as once in a great while there is a need to hide a field on a layout so that it is present but not visible...