This seems to be a bug... Might ought to Provide Motion Feedback...
That said, it's easy enough to get back. All you need to do is load up an image -- any image. Select the Drop Zone object layer in the Layers List and from the To drop down menu - select the new image. Then go right back to it and select the Drop Zone back. You can delete the image after that. Publish the Pan and Scale. {And do NOT hit the clear button or you'll have to start over.}
Another way to go is to import any image or make up a custom Drop Zone image (make one up and Export as a JPG or PNG). Load it into a Motion Project and from the Image inspector, select Drop Zone from the Type drop down menu. [The dimensions of the original image will play a role in how the drop zone image is displayed (cropped) in FCPX.]
Another trick would be to immediately create another new Drop Zone (do not duplicate the original). Then select the first one and select Drop Zone from the To menu. Delete the second Drop Zone you added.
HTH