Thank you for your reply, Don. Yes, I am well aware of the 4 GB size limit for TIFF files; I am not hitting that. The files I am generating are in the 2-2.8 GB range; starting with ~16 MP image files about 4600 px wide and upscaling 560-600% to reach 25,920px wide. That is 36" wide at 720 ppi, the latter number being the recommended input for Epson professional printers. I am evaluating whether AI Gigapixel can generate files of sufficient quality for printing at that size. My initial results suggest that it can, when starting with a high quality image file. I'm not the only one testing files of this size for this purpose, by the way.
I do not see the metadata problem for output files that are smaller than 2 GB. I'm not suggesting that it's a huge problem, because after Photoshop CC 2018 warns about the metadata it goes ahead and opens the file, and the image looks OK. But I would like to know what is causing it, and if it can be circumvented. I have tried changing a number of the parameters in AIG's dialog, without affecting the result. I'm using Windows 10 on a PC with relatively robust components, if that's important to know.
One of the forums where this behavoir has been confirmed is Luminous Landscape.