I had no clue the anchor strap was to go over top of the headrest. Seems like it would compress and crush it once you tightened.See manual. By design. Headrests are not adjustable. They can be removed but apparently should only be done by service. In the manual, they also tell you to strap a child seat’s rear anchor strap over the top of the headrest instead of through the sliver of an opening.
Most folks who have child seats will either just click the tabs in to move the headrest up or remove it altogether. Thought it was an asinine design but apparently it’s to meet safety standards.