I don’t know much about the block editor or FSE, I do not use them.
That said.
ACF must be able to determine the post to get the value from. If you put the shortcode in a block then I’m pretty sure the block ID will be used, but not sure.
If this is not block related then the shortcode, like any call to ACF functions, must be inside “The Loop”. This is the main loop for posts while (have_posts).... If where you are adding the shortcode is not in the loop then you must supply the post ID.
I do not know how you would do this in the editor you are using.
Perhaps someone that knows more about it will come along or you can try contacting ACF devs to see if they can give you any guidance.
