get_viewport().size returns window size not viewport

I am trying to port a project from Godot 3 to 4 atm

The projects "Viewport" (Just "Width" and "Height" under Godot 3.x) is set to 360x640, while the "Test Width/Height" (now called "Window Width/Height Override") is set to 720x1280

Under Godot 3.x get_viewport().size returns 360x640, whereas under Godot 4 Beta it returns the current window size

How do i get the right viewport size i set under the project settings?

Btw: Mode is set to viewport and Aspect is set to keep in both versions