Compatibility with WP Mail SMTP ?

Ok you are right. When the plugin was deactivated, the issue remained.

But I did find the issue, when I deactivated this snippet, the redirect worked again:

add_action( 'rwmb_frontend_after_process', function( $config, $post_id ) {
    if ( 'research-summary' === $config['id'] ) {
        wp_mail( '$myemail', 'Summary pending', 'A new research summary has been just submitted:
		Dashboard:$mydashboardlink 
		Google drive link: $linktoagoogledrive' ); //have replaced the links purposely with $ 
        die;
    }
}, 10, 2 );

Is it because this is overriding the action to redirect? And how would I fix this?

The redirect is in my shortcode, not in this code. But I will remove die;. Thank you.