What Does Gigapixel Save Images as TIFF?

I have Gigapixel set to save images as the same format as input but I find sometimes with already large images that i am upscaling it will save them as TIFF images instead even though it is set to save output as the same format as input. Does anyone know why it is doing this. For example when upscaling an 8000 x 5333 image to 16000 x 10666 it saves it as a TIFF image.

What is the format of the input image?

input is jpg

You will need to open a support request at the main website.