Nested Dictionary data lost after playmode

Hi there! I am very new to the subject of Serialization and I would like to save data from a "nested" dictionary, after I have added them in playmode.

I am trying to use Odin Serializer to save my data from a dictionary within a dictionary, but when I add a new dictionary inside the other dictionary at play mode, it is gone when I exit play mode again. The nested dictionary data is added when I use the function while I am in playmode.

Am I approaching it the right way, or do I need something else to keep this data after I close the session and the app?

Thanks a lot for your help!

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using Sirenix.OdinInspector;
 using Sirenix.Serialization;
  
  
 public class SavedFacialExpressions : SerializedMonoBehaviour {
  
     [DictionaryDrawerSettings(DisplayMode = DictionaryDisplayOptions.ExpandedFoldout)]
     [OdinSerialize]
     public Dictionary<string, Dictionary<string, float>> FacialExpressionDictionary;
  
  
 CharacterRig characterRig;
  
     void Start()
     {
         characterRig = GetComponent<CharacterRig>();
  
     }
  
     public void SaveExpression ()
     {
         ExpressionName = FacialExpressionDictionary.Count.ToString();
  
  
  
  
         FacialExpressionDictionary.Add (ExpressionName, new Dictionary<string, float>());
  
         FacialExpressionDictionary [ExpressionName].Add ("UpperEyeLidsPos", characterRig.UpperEyeLidsPos);
         FacialExpressionDictionary [ExpressionName].Add ("LowerEyeLidsPos", characterRig.LowerEyeLidsPos);
         FacialExpressionDictionary [ExpressionName].Add ("Squinch", characterRig.Squinch);
         FacialExpressionDictionary [ExpressionName].Add ("Wrath", characterRig.Wrath);
  
  
     }
        
 }