Unity custom editor losing values on Playmode

• I wonder why this happens on multi dimentional arrays.

  Because the

   public bool[,] b_slots2 = new bool[3,3];
  

  is not serializable by Unity (see Script Serialization Rules) so it can never be saved anyway!

  Note: Unity doesn’t support serialization of multilevel types (multidimensional arrays, jagged arrays, dictionaries, and nested container types). If you want to serialize these, you have two options: ...

  In your case you don't seem to really need those two options at all though. Simply make sure that you implement an according getter/setter method that handles your index conversion.

  This is not only affecting you when entering playmode! Even worse: All your settings would be lost when closing and opening Unity or in a build ;)

• In general: You go directly through the target which doesn't do any dirty state handling and therefor no saving, and no Undo/Redo either! You do this later on "manually" but I would always rather go through the serializedObject and SerializedPropertys which handle all the mentioned automatically!

• A minor one: For the slider you can directly use the [Range] attribute, no need for a custom editor code here.

So I would probably rather have e.g.

[CreateAssetMenu(fileName = "Block", menuName = "Scriptable/Data")]
public class StaticBlockData : ScriptableObject
{
    public BlockValue e_blockValue = 0;
    [Range(1, 9)] public int i_blockCount = 1;
    [SerializeField] private bool[] b_slots = new bool[9];

    public bool GetSlot(int x, int y)
    {
        return b_slots[GetFlatIndex(x, y)];
    }

    // if needed at all
    public bool SetSlot(int x, int y, bool value)
    {
        b_slots[GetFlatIndex(x, y)] = value;
    }

    public static int GetFlatIndex(int x, int y)
    {
        return x + (y * 3);
    }
}

and then

namespace FACustomEditor
{
    [CustomEditor(typeof(StaticBlockData)), CanEditMultipleObjects]
    public class BlockShapeCustomEditor : Editor
    {
        public override void OnInspectorGUI()
        {
            // little bonus for displaying the default script field ;)
            using (new EditorGUI.DisabledScope(true))
            {
                EditorGUILayout.ObjectField("Script", MonoScript.FromScriptableObject((StaticBlockData)target), typeof(StaticBlockData), false);
            }
        
            EditorGUILayout.Space();

            // loads current serialized values
            serializedObject.Update();

            // use the default drawers for the first two fields
            EditorGUILayout.PropertyField(serializedObject.FindProperty(nameof(StaticBlockData.e_blockValue)));
            EditorGUILayout.PropertyField(serializedObject.FindProperty(nameof(StaticBlockData.i_blockCount)));

            // So until here you actually didn't need any custom inspector

            // get the list property
            var b_slots = serializedObject.FindProperty("b_slots");

            EditorGUILayout.LabelField("Block Shape");

            // Create a boolean field
            for (var y = 0; y < 3; y++)
            {
                // you had a couple of nested horizontal and vertical scopes
                // actually all you need is a horizontal scope for each row (y)
                using (new EditorGUILayout.HorizontalScope())
                {
                    for (var x = 0; x < 3; x++)
                    {
                        var element = b_slots.GetArrayElementAtIndex(StaticBlockData.GetFlatIndex(x, y));
                        EditorGUILayout.PropertyField(element, GUIContent.none, GUILayout.MaxWidth(20));
                    }
                }
            }

            // writes back any changes and handles dirty marking, undo/redo and saving
            serializedObject.ApplyModifiedProperties();
        }
    }
}

And assuming the i_blockCount is actually just the amount of enabled slots you could as well let your inspector process it as well and display it as read-only

public override void OnInspectorGUI()
{
    using (new EditorGUI.DisabledScope(true))
    {
        EditorGUILayout.ObjectField("Script", MonoScript.FromScriptableObject((StaticBlockData)target), typeof(StaticBlockData), false);
    }

    EditorGUILayout.Space();

    serializedObject.Update();

    EditorGUILayout.PropertyField(serializedObject.FindProperty(nameof(StaticBlockData.e_blockValue)));

    var blockCount = serializedObject.FindProperty(nameof(StaticBlockData.i_blockCount));

    // draw as read-only -> User can not change manually
    using (new EditorGUI.DisabledScope(true))
    {
        EditorGUILayout.PropertyField(blockCount);
    }

    var b_slots = serializedObject.FindProperty("b_slots");

    EditorGUILayout.LabelField("Block Shape");

    // initially reset to 0
    blockCount.intValue = 0;

    for (var y = 0; y < 3; y++)
    {
        using (new EditorGUILayout.HorizontalScope())
        {
            for (var x = 0; x < 3; x++)
            {
                var element = b_slots.GetArrayElementAtIndex(StaticBlockData.GetFlatIndex(x, y));
                EditorGUILayout.PropertyField(element, GUIContent.none, GUILayout.MaxWidth(20));

                // increase for each ticked slot
                if (element.boolValue)
                {
                    blockCount.intValue++;
                }
            }
        }
    }

    serializedObject.ApplyModifiedProperties();
}