[FIX] Markdown not rendering as preview for threaded messages (#3465)

* Use older preview logic

* Stories

__tests__/__snapshots__/Storyshots.test.js.snap

@@ -18121,6 +18121,26 @@ exports[`Storyshots Markdown Preview 1`] = `
   >
     Testing: 😃 👍 :marioparty:
   </Text>
+  <Text
+    accessibilityLabel="Fallback from new md to old"
+    numberOfLines={1}
+    style={
+      Array [
+        Object {
+          "backgroundColor": "transparent",
+          "fontFamily": "System",
+          "fontSize": 16,
+          "fontWeight": "400",
+          "textAlign": "left",
+        },
+        Object {
+          "color": "#2f343d",
+        },
+      ]
+    }
+  >
+    Fallback from new md to old
+  </Text>
 </View>
 `;
 

app/containers/markdown/index.tsx

@@ -369,7 +369,7 @@ class Markdown extends PureComponent<IMarkdownProps, any> {
 			return null;
 		}
 
-		if (this.isNewMarkdown) {
+		if (this.isNewMarkdown && !preview) {
 			return (
 				<NewMarkdown
 					username={username}

storybook/stories/Markdown.js

@@ -80,6 +80,24 @@ stories.add('Preview', () => (
 			username='rocket.cat'
 		/>
 		<Markdown msg='Testing: 😃 :+1: :marioparty:' getCustomEmoji={getCustomEmoji} theme={theme} numberOfLines={1} preview />
+		<Markdown
+			msg='Fallback from new md to old'
+			getCustomEmoji={getCustomEmoji}
+			theme={theme}
+			numberOfLines={1}
+			preview
+			md={[
+				{
+					type: 'PARAGRAPH',
+					value: [
+						{
+							type: 'PLAIN_TEXT',
+							value: 'This is Rocket.Chat'
+						}
+					]
+				}
+			]}
+		/>
 	</View>
 ));