Recently the interior humidity was below 10% almost all the time. In that situation I will get a highly predictable left-right head swipe exactly 8 inches in from the leading edge of the roll paper, if the printer has stood idle for more than about 1 hour. Then no more swipes for the rest of the print, no matter how long.
However, with the evaporative cooler running the inside humidity now stays at about 30%, and the problem is no longer present.
The paper path adds a nasty curve to the paper at that position, similar to what I see the pictures above. Too much idle time and too little humidity, and the paper takes a curved set. The fix was to roll off 8 inches before starting, or even better to do a lot of printing at once. For really thick, stiff media, I also cut off most the 8" advance. I think the problem would be primarily with very stiff media, since I never saw the problem with limp canvas, only with rather stiff canvas like glossy canvas.
FWIW, be sure you're upgraded to printer firmware 1.26, which seems to be addressed to printhead issues. Since I did that I have had no issues beyond the swipes, even though both my heads are near the 1 teradot mark.