add_action(‘the_content’, ‘my_plugin_content’) is null

I am creating in personal plugin where I recover data from a personal table.
Con add_action(‘the_content’, ‘my_plugin_content’); posso far vedere il contenuto in una pagina.
I would like to save the content in wp_posts with add_filter (‘save_post’, ‘my_plugin_content’), but the content is empty.

in another file I run the data recovery query that I refer to the file loader that loads the views

File loader

File view