the_content() not showing the content in good order (with a shortcode)

Question

I've added a shortcode [form] inside my wysiwyg the 2 paragraphes before the shortcode.

<p>test 1</p>

<p>test2</p>

[form]

but on the frontend, the shortcode is showing at the top of the content. I thought it was an error in CSS, but the HTML is indead rendering the form, and then my paragraph.

Any idea? I'm on WP 5.4

EDIT :

This is how the shortcode is created :

/* SHORTCODE */
function form_shortcode() {
    include( "includes/form.php" );
    return;
}

add_shortcode( 'form_calcul', 'form_shortcode' );

I have a few lines of php inside. I should probably precise, I'm in a plugin. I tried also this, but no success:

    ob_start();
get_template_part('form');
return ob_get_clean();

Answer 1

Before the_content is displayed WP looks for filters to run the content through. One of those filters is do_shortcode. The idea of a filter is that you can change something before it is displayed. As a consequence, if inside the filter function you echo something, it ends up before the_content, which is still waiting for the filtering process to be completed.

Since your function form_shortcode() does not return a modified content anything that is displayed must be the result of echo statements in form.php.