there’s almost a 10mm gap
It looks to me like the rubber meets well at the top? Perhaps it also meets well at the bottom, but that is out of shot.. But it looks like the opening window frame has bowed away from the part it's supposed to meet with in the middle
If you put a straight edge/spirit level etc on the moving part or the fixed part of the window you'll find one of them is bent like a banana.. Yep, that's PVC for you.. The longer the section the more likely it is to bend in the heat of the sun etc
To prevent this windows are usually fitted with a device that stabilises/locks into place at regular intervals along a long run. The plastic frame may also have a metal insert placed inside it at the factory. Wouldn't be the first time that one or the other of these things has been forgotten to be fitted
Here is an example:
See the white/silver thing? When the window closes this pulls the moving and fixed frame together preventing one from bowing away from the other. On the other side of a window something like this may be present along a long run of framework:
On the right is a big silver lump with slots in and a visible screw head. On the other side is a small silver mushroom shape visible against the backdrop of the black rubber seal. The silver mushroom slides into the slots in the big silver thing when the handle is turned. When placed mid run this device clamps the window shut and keeps the frame straight. These are only effective if the mushroom actually slides into the slot; I've had to repair one of my windows where the side was bowed and letting noise in because the frame had pulled away from the opener and the mushroom missed the slot
Neither of these things can be fitted to the sides of a top hung window with long sides unless there is a cable and gearbox system inside the frame like tilt n turn windows have
It's not clear where the handle on your window is, nor where the hinges are. Take some more pictures of the window open, and of any metal or plastic lumps you see screwed to the sides of the window when it's open