When the content is filtered with the_content filter, the replaced content is truncated because it's not wrapped in the post-content block.

Screenshots of the issue

On Five for the Future we inject the company description:

In "notice" blocks, we run the_content to parse markup:

To reproduce

• Add the following to your site, which will replace all content with 123456789

   add_filter(
   	'the_content',
   	function() {
   		return '123456789';
   	},
   	1
   );
  

• View any page, and you'll only see 456.

More details

This is due to #67272 adding add_filter( 'the_content', 'remove_serialized_parent_block', 8 );, and remove_serialized_parent_block does not check for serialized blocks before running substr. This results in it trimming from 3, -3 if there is no wrapper.

Additionally, any blocks that run the_content on attributes are also truncated, as this filter runs in the course of rendering the block.

I'm not sure if the better solution is to add a check in remove_serialized_parent_block, or to change how the filter is applied in core/post-content, but I'm happy to draft a core patch if needed.