The crease is from the edge of the RC paper putting pressure on the print as you roll it. If you dry mount the print, this
mark will not show.
I don't know of anyway not to get this effect, rolling from the side still causes it for me. I have had some luck using a sheet
of Epson Enhanced matte paper over the print. This seems to lessen the effect of the edge pressure. If the De-Roller had a
softer material to help absorb some of the pressure, then this might not be a problem.
Fine art papers don't seem to have this issue as you pointed out,
Paul C