I'm trying to save the initial value (saved_fname) of a new user created, when the registration is done from the admin panel it works but when I do it from REST API (Integromat) the user registers but the function doesnt work correctly. Same code, with no plugins installed.

Both methods, the function "saved_first_name" is executed but using REST API it is as if the values in the wp_usermeta table have not been registered yet, get_user_meta returns empty.

functions.php theme
function saved_first_name($user_id){
    $saved_fname = get_user_meta($user_id, 'first_name', true);
    update_user_meta($user_id, 'saved_fname', $saved_fname);
    update_user_meta($user_id, 'imhere', 'yes');
}

add_action( 'user_register', 'saved_first_name' );

DB usermeta table result when registering user from admin panel:

• metakey: metavalue
• imhere: yes
• saved_fname: user1
• first_name: user1
• nickname: user1

DB usermeta table result when registering user from API REST:

• metakey: metavalue
• imhere: yes
• saved_fname: (empty-no value)
• first_name: user2
• nickname: user2

I can't find the reason for that behavior. Any suggestions are welcome & appreciated.