Why is my serialized custom class/struct not properly instantiated (values return null/default) ?

Problem in a nutshell:

I have created a custom class/struct and made it [Serializable], then added [SerializeField] properties to access its field in the editor. In-editor this behaves as excepted, however on run properties referencing Objects either return null or the default value.

Code:

 public class PlayerCollisionChecks : MonoBehaviour {
 
 [Serializable] public struct RaycasterGroup { // Using class makes no difference
         public CollisionReport collisionType;
         public LayerMask collisionMask;
         public Transform startPoint;
         public Transform endPoint;
         public RaycastDirection direction;
         public float distance;
         public int numberOfRays;
         public bool showDebugRays;
     }
 
 [SerializeField] RaycasterGroup _firstGroup;
 
 ...
 
 // returns
 // start- & Endpoint -> always null
 // CollisionType & Direction (custom Enums) -> always first(=default) value in grouping
 // distance & number of rays (int) -> the value set in the editor (as expected)
 }

As far as I can tell I'm following the guidelines outlined here. I've tried changing it to a class and using ISerializationCallbackReceiver, but that seems to prevent serialization the way I intend it (i.e. as dropdown menus, see picture).