Steps to reproduce:

1. Create a new post with the block editor.
2. Add a heading block.
3. Add a few paragraph blocks.
4. Go to More > Preview.
5. Check to see if the heading appears as expected.

Result: headings aren't displayed correctly in previews.

Editor view:	Preview:
In-app browser:	Chrome view:

_{Tested with WPAndroid alpha-171 on Pixel 3 Android 9.}