diff --git a/gatsby-config.js b/gatsby-config.js
index 773e33bffba3..c6fab89b8e09 100644
--- a/gatsby-config.js
+++ b/gatsby-config.js
@@ -14,7 +14,7 @@ module.exports = {
   flags: {
     FAST_DEV: true,
     PARALLEL_SOURCING: true,
-    PRESERVE_FILE_DOWNLOAD_CACHE: true,
+    // PRESERVE_FILE_DOWNLOAD_CACHE: true,
     DETECT_NODE_MUTATIONS: true,
     PARTIAL_HYDRATION: true
   },
diff --git a/gatsby-node.js b/gatsby-node.js
index 9f44ad37dbb9..c38e2531ceed 100644
--- a/gatsby-node.js
+++ b/gatsby-node.js
@@ -905,15 +905,6 @@ exports.onCreateNode = ({ node, actions, getNode }) => {
         onCreateChapterNode({ actions, node, slug });
         return;
       }
-
-      if (node.internal.type === "Mdx") {
-        if (!node.frontmatter.thumbnail) {
-          node.frontmatter.thumbnail = null;
-        }
-        if (!node.frontmatter.darkthumbnail) {
-          node.frontmatter.darkthumbnail = null;
-        }
-      }
     }
   }
 };