Custom inspector variable resets automatically

I am trying to make a custom script that allows to spawn prefabs from the editor (without enetering play mode). In this editor there is a field like this:

 Vector3 spawnPosition = Vector3.zero;

Whenever I change the focus on the script, or I click play or stop, the field gets reset.

I have been looking on the internet and the clasic solutions doesn't seem to work anymore. Examples:

• Adding [SerializeField] to the variable

• Adding [System.Serializable] the the class

• Removing = Vector3.zero; from the declaration

• Invoking EditorUtility.SetDirty(target);

• Since the variables are from the custom editor and not the script, this doesn't work neither: SerializedProperty

The class works fine but the custom variables get reset all the time which is very anoying.

Did anyone else run into the same problem? Thanks