I'm having some troubles with Prusament Galaxy Black PETG. It prints perfectly, but when printing large(r) objects that span the whole width of the bed I'm getting matte spots near the ends:

I've tried slowing the top layer WAY down to 35mm/s but that was even worse with what seems over extrusion and random bits of fillament poking out, and still: matte spots. I'm printing in an enclosure using this 3MF file: https://drive.google.com/file/d/1ROK2_4f2W_0N4vmkt1mnXg3wmzp4NLQQ/view?usp=sharing

When sliced I'm not seeing slowdown, temp changes, cooling changes, flow changes etc. Only the actual volumetric flow rate shows SOME changes compared to the rest of the top layer in the corners. But, those don't cover the matte spots.