So this is new to me so I am willing to only venture a guess and suggest that changing a few things with your setup to see if the results might be different.
First, it looks (not trying to be too obvious here) as an edge effect. I wonder if changing the mount tissue would change this appearance of these lines. It may be due to the papers dimensional stability under the heat.
I might suggest a couple of other things. First, I would try just one sheet of 4 ply or even 2 ply. Heating so much paper takes a lot of energy. If you only used one sheet, you could press for less time. I also think that the moisture in the paper would have less medium to travel through to be released.
My setup uses as little material between the thing to be heated and the platen. I also use a lot of release paper, a silicone impregnated paper that allows one to easily remove the mount tissue from the paper.
I use the press to attach tissue paper via 1 sheet of release paper, the print, upside down, tissue, one more sheet of release paper or release board (about 4 ply) and that contacts the platen. I then let it cool and trim the print, I then set the print on the board where I want it, put it back in the press with the print facing up and place release paper or board on top which contacts the platen.
I keep in under for 45 seconds at 225 F. I keep the platen clean with naphtha and use only the smoothest surface to come in contact with the print as I have left texture in the print in the past by choosing a bad surface covering the print.
I hope this helps in some small way.