Output hooked to the_content is output after template parts in Block Based Themes

[johnstonphilip]

Describe the bug
If you hook extra output to the_content, like many plugins do, template_parts in block-based themes show that output after the template part is shown.

To reproduce
Steps to reproduce the behavior:

1. Create a block-based theme, with the header.html and footer.html in the block-template-parts directory, and the index.html file pulling those in.
2. Add this code to a custom plugin:

function my_content_after_the_content( $the_content ) {
	return $the_content .
	'<div class="sharing_icons">I am hooked to the_content, to appear after every output of the_content</div>';
}
 add_filter( 'the_content', 'my_content_after_the_content' );

3. View a post.
4. Notice you see the hooked output after each template_part is shown.

Expected behavior
Things hooked to show after the_content only show after the_content of a post.

Desktop (please complete the following information):

• OS: MacOS
• Browser: Chrome Version 79.0.3945.130 (Official Build) (64-bit)

The use of this filter code should probably be removed:

gutenberg/packages/block-library/src/template-part/index.php

Line 35 in 21304f9

return apply_filters( 'the_content', str_replace( ']]>', ']]>', $content ) );

[johnstonphilip]

I opened a PR for this here:
#20343