fix(v2): fix multi-instance mdx loaders not sandboxed correctly (#3970)

* another attempt to fix the multi-instance conflicts

* try to fix windows build

* cleanup + improve comment

packages/docusaurus-plugin-content-blog/src/index.ts

@@ -15,6 +15,7 @@ import {
   getPluginI18nPath,
   reportMessage,
   posixPath,
+  addTrailingPathSeparator,
 } from '@docusaurus/utils';
 import {
   STATIC_DIR_NAME,
@@ -436,7 +437,9 @@ export default function pluginContentBlog(
           rules: [
             {
               test: /(\.mdx?)$/,
-              include: getContentPathList(contentPaths),
+              include: getContentPathList(contentPaths)
+                // Trailing slash is important, see https://github.com/facebook/docusaurus/pull/3970
+                .map(addTrailingPathSeparator),
               use: [
                 getCacheLoader(isServer),
                 getBabelLoader(isServer),