The error occurs when I nest multiple subform fields.
I wrote the following code to fix the issue:

// Flatten the value if it is an array (list, checkboxes, etc.), independent of render_values
if (is_array($subfield->value)) {
    if (!empty($subfield->value)) {
        $new_value = "";

        array_walk_recursive($subfield->value, function ($value) use (&$new_value) {
            $new_value .= $value . " ";
        });

        $subfield->value = trim($new_value);
    } else {
        $subfield->value = implode(' ', $subfield->value);
    }
}