Custom Shortcode's Output Isn't Contained by

This is the simplest example of the problem.

function welcome_user(){
	$current_user = wp_get_current_user();
	
	if( $current_user -> ID == 0){ //if no one is logged in, redirects to login page
		?>
		<META HTTP-EQUIV="Refresh" CONTENT="0;URL=index.php/login">
		<?php
	} else {
		echo'<H1>Welcome ' . $current_user -> user_firstname . '!</H1>';
	}
}

This is saved in the wp-content folder as a file named the same as the function. In the functions.php file I have:

include(WP_CONTENT_DIR . '/welcome_user.php');
add_shortcode('welcome_user', 'welcome_user');

Then I added the shortcode to the page. I’ve tried both the textbox and the code block. Both work when it comes to adding the content, except that it loads the shortcodes content before it adds anything else in the avia layout builder.