[suii]

Add is_checkout() on your condition then change the fields to billing_address_1 and 2.

add_filter( 'woocommerce_checkout_fields', 'readonly_billing_account_fields', 25, 1 );
function readonly_billing_account_fields ( $billing_fields ) {
    // Only my account billing address for logged in users
    if( (is_user_logged_in() && is_account_page()) || is_checkout()){

        $readonly = ['readonly' => 'readonly'];

        $billing_fields['billing']['billing_address_1']['custom_attributes'] = $readonly;
        $billing_fields['billing']['billing_address_2']['custom_attributes'] = $readonly;
    }
    return $billing_fields;
}

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