Peter,
I believe what you are seeing are what I used to call "pressure marks" They are created by the canvas being pulled over the platen. When you look at the platen, note there are raised parts, or bars and valleys. I believe your lines will line up with the raised bars. BTW look on the back of any paper print especially RC and you may also see the same marks running down the back of the print.
Personally, I have always been surprised more people didn't notice this as all my 9880's did it to some degree. My 3 9880 which was an Epson replacement and was shipped as a new machine made the marks, but over time they would fade out. Epson will tell you that this is normal and within spec, as I had 3 or 4 open calls on it for over a year.
IMO it's tied to the suction i.e. the fans but there is no control for this. Yes, you can set suction when you load canvas, (increase or decrease the amount) but as soon as the print starts, the suction returns to a normal setting that you can't control.
For the most part for me, the lines will fade out over time or once the canvas is coated and stretched you will not see them. I hope yours work the same way.
For the record, my 9900 does the same thing with glossy paper, and faintly on canvas. I have only found two glossy papers I can run that the 9900 will not "indent" with the same lines. It's not a problem with luster. On Canvas I may see the lines, but they are very faint and fade out by the time I coat. With fibre paper especially Canson or Museso, I will also see the marks but since I dry mount paper prints (due to humidity in my location) they are not visible on the final product.
Upon my install of the 9900 and my discovery of the lines/indents, I immediately opened a new call with Epson and shipped them several prints showing the effect. Again their response was "normal" and "within spec" and they also claimed that they got the same marks with their lab 9900. Personally I was surprised by this but it is what it is, and thus I started testing papers and canvas to see which would preform better.
Paul Caldwell