CustomEditor: Inspector icon rapidly switching

I made a simple CustomEditor-script just to hide and show inspector parameters based on the current state of the MonoBehaviour.

The Problem is the moment I add that script to a GameObject the inspector icon starts switching between the attached sprite and the default Prefab-icon.

I don't set the icon in the CustomEditor-script, so it baffles me why this happens only when the script with the CustomEditor is attached.

Google didn't help so I'm hoping to find help here, of which any is greatly appreciated.

Edit: As requested, here is the code (if it is too long I will gladly shorten it)

 using UnityEngine;
 using UnityEditor;
 
 [CustomEditor(typeof(PlayerMovement)), CanEditMultipleObjects]
 public class PlayerMovementEditor : Editor
 {
     public SerializedProperty
         mode_p,
         physics_p,
         accelerationRate_p,
         breakRate_p,
         maxSpeed_p,
         accelerationForce_p,
         turnSpeed_p;
         
     private void OnEnable() {
         // Setup SerializedProperties
         mode_p = serializedObject.FindProperty("mode");
         physics_p = serializedObject.FindProperty("physics");
         accelerationRate_p = serializedObject.FindProperty("accelerationRate");
         breakRate_p = serializedObject.FindProperty("breakRate");
         maxSpeed_p = serializedObject.FindProperty("maxSpeed");
         accelerationForce_p = serializedObject.FindProperty("accelerationForce");
         turnSpeed_p = serializedObject.FindProperty("turnSpeed");
     }
 
     public override void OnInspectorGUI() {
         serializedObject.Update();
 
         // enums can't be changed when moving during runtime
         GUI.enabled = !Application.isPlaying || !(target as PlayerMovement).IsMoving;
         EditorGUILayout.PropertyField(mode_p);
         EditorGUILayout.PropertyField(physics_p);
         GUI.enabled = true;
 
         PlayerMovement.Mode mode = (PlayerMovement.Mode)mode_p.enumValueIndex;
         PlayerMovement.PhysicsLevel physics = (PlayerMovement.PhysicsLevel)physics_p.enumValueIndex;
 
         switch (physics)
         {
             case PlayerMovement.PhysicsLevel.None:
                 EditorGUILayout.PropertyField(maxSpeed_p, new GUIContent("Max Speed"));
                 break;
             case PlayerMovement.PhysicsLevel.RateBased:
                 EditorGUILayout.PropertyField(maxSpeed_p, new GUIContent("Max Speed"));
                 EditorGUILayout.Slider(accelerationRate_p, 0f, 1f, new GUIContent("Acceleration Rate"));
                 EditorGUILayout.Slider(breakRate_p, 0f, 1f, new GUIContent("Break Rate"));
                 break;
             case PlayerMovement.PhysicsLevel.ForceBased:
                 EditorGUILayout.PropertyField(accelerationForce_p, new GUIContent("Acceleration Force"));
                 break;
         }
 
         if (mode == PlayerMovement.Mode.Ship)
             EditorGUILayout.PropertyField(turnSpeed_p, new GUIContent("Turn Speed"));
 
         serializedObject.ApplyModifiedProperties();
     }
 }