There\'s a few ways to achieve this but here\'s one which supports an unknown depth of children, for example where you have nested groups within groups.

Given the field key "field_{some_hash}" of the field, this function will retrieve the field, and its parent most field, and so on until we reach the top of the field tree.

function custom_acf_get_field_name_by_key( $key ) {

    $field = acf_maybe_get_field( $key );

    if ( empty( $field ) || ! isset( $field[\'parent\'], $field[\'name\'] ) ) {
        return $field;
    }

    $ancestors = array();

    while ( ! empty( $field[\'parent\'] ) && ! in_array( $field[\'name\'], $ancestors ) ) {

        $parent = acf_get_field( $field[\'parent\'] );

        $ancestors[] = $field[\'name\'];

        $field = $parent;

    }

    $formatted_key = array_reverse( $ancestors );
    $formatted_key = implode( \'_\', $formatted_key );

    return "_$formatted_key";

}

Assume a case where I have a nested group:

 - dev_group_test | field_61ecf308ded3e | group
   ∟ title        | field_61ecf316ded3f | text
   ∟ location     | field_61ecf31cded40 | text
   ∟ sub_group    | field_61ecfa1fe5894 | group
     ∣
     ∟ sub_group_title | field_61ecfa2de5895 | text

If I want the full key of sub_group_title:

$meta_key = custom_acf_get_field_name_by_key( \'field_61ecfa2de5895\' );
// result = _dev_group_test_sub_group_sub_group_title

If I want the full key of location:

$meta_key = custom_acf_get_field_name_by_key( \'field_61ecf31cded40\' );
// result = _dev_group_test_location

NOTE: there may be more efficient ways to do this