My custom AssetPostprocessor.OnPreprocessTexture() logic is being run every time my project is saved

I used this tutorial to get some default import settings working, but I can no longer change any setting that is defined in my custom import class. The moment I hit "Apply," the class is called and the new values get reset back to the custom defaults. I just want this to be run the very first time an asset is imported, not constantly.

Here is my version of the code:

 // -------------------------------------------------------------------
 // - A custom asset importer for Unity 3D game engine by Sarper Soher-
 // - http://www.sarpersoher.com                                      -
 // -------------------------------------------------------------------
 // - This script utilizes the file names of the imported assets      -
 // - to change the import settings automatically as described        -
 // - in this script                                                  -
 // -------------------------------------------------------------------
 
 
 using UnityEngine;
 using UnityEditor;  // Most of the utilities we are going to use are contained in the UnityEditor namespace
 
 // We inherit from the AssetPostProcessor class which contains all the exposed variables and event triggers for asset importing pipeline
 internal sealed class CustomAssetImporter : AssetPostprocessor {
 
     #region Methods
 
     //-------------Pre Processors
 
     // This event is raised when a texture asset is imported
     private void OnPreprocessTexture() {
         // I prefix my texture asset's file names with tex, following 3 lines say "if tex is not in the asset file name, do nothing"
         var fileNameIndex = assetPath.LastIndexOf('/');
         var fileName = assetPath.Substring(fileNameIndex + 1);
 
         if(!fileName.Contains(".png")) return;
 
         // Get the reference to the assetImporter (From the AssetPostProcessor class) and unbox it to a TextureImporter (Which is inherited and extends the AssetImporter with texture specific utilities)
         var importer = assetImporter as TextureImporter;
         
         importer.spritePixelsPerUnit = 32;
         importer.filterMode = FilterMode.Point;
         importer.textureCompression = TextureImporterCompression.Uncompressed;
     }
 
     // This event is raised every time an audio asset is imported
     // This method does nothing at the moment, just a skeleton to fill in if we ever need to do audio specific importing
     // Imports audio assets in the default way without changing anything
 //    private void OnPreprocessAudio() {
 //        var fileNameIndex = assetPath.LastIndexOf('/');
 //        var fileName = assetPath.Substring(fileNameIndex + 1);
 //
 //        if (!fileName.Contains("snd")) return;
 //
 //        var importer = assetImporter as AudioImporter;
 //        if(importer == null) return;
 //    }
 
     #endregion
 }