My guess is that the .umap file stores the Transform information of your camera component, so even the default value had changed it was override by the value from the map.
Two possible fix:
Easy way (Not recommended): Set the camera rotation in BeginPlay().
This expose the camera to the Blueprint. After compiling, create a BP version of the pawn and click “OurCamera” component. Now you can adjust the location and rotation at the “Transform” section. This is better workflow in larger projects because you can see the result in real-time.
What you entered in SetRelativeRotation() is just for default value.