Sagging walls

I’ve got an issue with my walls on this print. The part is quite thin but I’ve never seen this before regardless. What could be the issue? I’ve checked layer speed, which is the same across the entire wall surface. And layer time which does not apear to change that much up the surface.

My thaught was a differance in cooling time between layers. I’m useing Creality hyper pla with a dryer so that should not be the issues.

I know the bed, which I have set to 60 C radiates heat a certain distance from is. Could the drop in air temperature around the print, caused by a greater distance from the build plate have anything to do with it?

Thanks!