what is returned is not what you think it is in this case
https://developer.wordpress.org/reference/functions/update_post_meta/
Return
int|bool Meta ID if the key didn’t exist, true on successful update, false on failure or if the value passed to the function is the same as the one that is already in the database.
Since you are passing the same value that is already set it is returning false. This is what ACF returns. The purpose of what you are doing is to force ACF to set the field key reference.
