When I run the shortcut, I get a PNG image in DTTG. This, of course, does never have any metadata, so it is no wonder that you do not see any (in my case, even the size in Bytes is given as “0” although the image is several MB in size). But, as you described, changing the entry for UTI to “public.jpeg” gives me a JPEG – again with size 0 and no metadata.
However, if I share an image from photos lib to DTTG, I get the metadata and size just fine. So I suppose that something™ goes awry in the “create object in DDTG” action when the object is an image. @BLUEFROG: Do you have any insight into this?