Hi,
It works perfectly as you said. If you choose Render for: Only the post content area and Position: Replaces the post content, on the tag archive page, other elements of the theme support like featured image, post title, date, author ... still keep up and only post content will be replaced.
If you want to replace the main content area with the grid and keep the sidebar, you can use Render for: The layout between header and footer and add this code to show the sidebar as well {{ mb.get_sidebar() }}.
Or use Type: Custom (via Shortcode) then add the shortcode [ mbv id="12345" ] to your Grid plugin.
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