When I originally looked on my phone, I couldn't tell where the picture was actually taken. Looking on my PC, I can see the rear seats so presume the problem is along the top of the tailgate.
O/T I see you have the same car seat us us. Have you got a seat cover? Doesn't look like it. We've got one of these and it kept our D4 seats pristine.
https://www.amazon.co.uk/Britax-2000009538...B00COFVCYW
Are the plastic 'plugs' that hold the head lining in place properly pushed in? Give the headlining a thump with the heel of your wrist a few inches forwards of the trailing edge and see if that raises the lining more permanently. You can see where these 'plugs' are if you pull the headlining down a bit.
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