Gutenberg block with clone field doesn't show default value

When I add a block in the Gutenberg editor that contains a "clone" field, its value is shown only when I change something in any field.

ACF Group fields:

1. section_appearance (group), subfields:

• background (select, defalut: "white")
• bottom_border (true/false, default: false)

2. section, fields:

• appearance (clone of the "section_appearance", display: Seamless)

Template added with the acf_register_block function:

echo "appearance:\n";
print_r(get_field('appearance'));
echo "section_appearance:\n";
print_r(get_field('section_appearance'));
?>

What I see after adding a block in the editor:

appearance:
section_appearance:
Array
(
    [field_60fe956ea9e15] => white
    [field_60fe95cda9e16] => 0
)

What I see when I change something in any input and after block view is refreshed:

appearance:
Array
(
    [section_appearance] => Array
        (
            [background] => white
            [bottom_border] => 
        )

)
section_appearance:
Array
(
    [background] => white
    [bottom_border] => 
)

Versions:
WP 5.7.2
ACF Pro 5.9.8

Start by reproducing the issue with the acf_register_block template and the listed ACF Group, clone, select, and true/false fields on WordPress 5.7.2 with ACF Pro 5.9.8. Compare the initial block render with the render after changing an input; done means the clone field returns and displays its default values immediately after the block is added.