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-docs/src/index.ts

@@ -17,6 +17,7 @@ import {
   aliasedSitePath,
   reportMessage,
   posixPath,
+  addTrailingPathSeparator,
 } from '@docusaurus/utils';
 import {LoadContext, Plugin, RouteConfig} from '@docusaurus/types';
 
@@ -345,7 +346,9 @@ export default function pluginContentDocs(
       function createMDXLoaderRule(): RuleSetRule {
         return {
           test: /(\.mdx?)$/,
-          include: flatten(versionsMetadata.map(getDocsDirPaths)),
+          include: flatten(versionsMetadata.map(getDocsDirPaths))
+            // Trailing slash is important, see https://github.com/facebook/docusaurus/pull/3970
+            .map(addTrailingPathSeparator),
           use: compact([
             getCacheLoader(isServer),
             getBabelLoader(isServer),