get_post_meta returns a bool (false) calling post from Ajax

There is a function that fixes the problem of displaying bool (false) when I call get_post_meta with Ajax. Bool (false) display with a var_dump Otherwise I have nothing.

I only get this problem with Ajax. With my single.php the get_post_meta is displayed.

I have no error, just "bool (false)".
Do you know where the problem comes from?