PayPal Standard with IPN: NOT Checking if IPN response is valid

Hi,
In my eCommerce some Payements work correctly and some others not…
I already search a lot of similar post in this and other forums but I haven’t found a solution.
My issue is very very similar to that one on GitHub: https://github.com/woocommerce/woocommerce/issues/16950 but there isn’t a solution…

First of all I post the following case where all works fine (my customer can pay with payPal) and the debug log are like this:

2021-04-18T17:17:10+00:00 INFO Generating payment form for order 15723. Notify URL: https://www.studio54ltd.com/wc-api/WC_Gateway_Paypal/
2021-04-18T17:17:10+00:00 INFO PayPal Request Args for order 15723: Array
(
    [cmd] => _cart
    [business] => *******@alice.it
    [no_note] => 1
    [currency_code] => EUR
    [charset] => utf-8
    [rm] => 2
    [upload] => 1
    [return] => https://www.studio54ltd.com/cassa/order-received/15723/?key=wc_order_************&utm_nooverride=1
    [cancel_return] => https://www.studio54ltd.com/cart/?cancel_order=true&order=wc_order_********&order_id=15723&redirect&_wpnonce=*************
    [image_url] => https://www.studio54ltd.com/wp-content/uploads/2019/12/LogoPerPayPal.jpg
    [paymentaction] => authorization
    [invoice] => WEB-15723
    [custom] => {"order_id":15723,"order_key":"wc_order_***********"}
    [notify_url] => https://www.studio54ltd.com/wc-api/WC_Gateway_Paypal/
    [first_name] => ***
    [last_name] => ***
    [address1] => ***
    [address2] => ***
    [city] => ***
    [state] => ***
    [zip] => ***
    [country] => ***
    [email] => ***@***
    [night_phone_a] => ***
    [night_phone_b] => ***
    [address_override] => 0
    [no_shipping] => 0
    [item_name_1] => Nike Air Force Shadow - 40 x 1
    [quantity_1] => 1
    [amount_1] => 139
    [item_number_1] => 15723
    [bn] => WooThemes_Cart
)

2021-04-18T17:23:34+00:00 INFO Checking IPN response is valid
2021-04-18T17:23:36+00:00 INFO IPN Response: Array
(
    [headers] => Requests_Utility_CaseInsensitiveDictionary Object
        (
            [data:protected] => Array
...
...
VERIFIED
...
...
2021-04-18T17:23:36+00:00 INFO Received valid response from PayPal IPN
2021-04-18T17:23:36+00:00 INFO Found order #15723
2021-04-18T17:23:36+00:00 INFO Payment status: pending

Afted that I accept the payement in WooCommerce and the status correctly canged.
SO I THINK that all works correctly in PayPal and WooCommerce configuration.

BUT… Sometime after the customers pay I receive only the POST message from PayPal but seem that WooCommerce not send back Response to PayPal to confirm IPN and so the payment process not continue… and from the customers point of view nothing happens… And my order still “Waiting Payement”…

If I set “hold stock time” in WooCommerce settings, after that time the order automatically set to “Cancelled“.

How can I resolve?

Here there is the log message:

2021-04-19T21:19:21+00:00 INFO Generating payment form for order 15731. Notify URL: https://www.studio54ltd.com/wc-api/WC_Gateway_Paypal/
2021-04-19T21:19:21+00:00 INFO PayPal Request Args for order 15731: Array
(
    [cmd] => _cart
    [business] => [email protected]
    [no_note] => 1
    [currency_code] => EUR
    [charset] => utf-8
    [rm] => 2
    [upload] => 1
    [return] => https://www.studio54ltd.com/cassa/order-received/15731/?key=wc_order_m08ftXUIsVJvo&utm_nooverride=1
    [cancel_return] => https://www.studio54ltd.com/cart/?cancel_order=true&order=wc_order_m08ftXUIsVJvo&order_id=15731&redirect&_wpnonce=77724cc119
    [image_url] => https://www.studio54ltd.com/wp-content/uploads/2019/12/LogoPerPayPal.jpg
    [paymentaction] => authorization
    [invoice] => WEB-15731
    [custom] => {"order_id":15731,"order_key":"wc_order_m08ftXUIsVJvo"}
    [notify_url] => https://www.studio54ltd.com/wc-api/WC_Gateway_Paypal/
    [first_name] => ***
    [last_name] => ***
    [address1] => ***
    [address2] => ***
    [city] => ***
    [state] => ***
    [zip] => ***
    [country] => ***
    [email] => ***@***
    [night_phone_a] => ***
    [night_phone_b] => ***
    [address_override] => 0
    [no_shipping] => 0
    [item_name_1] => Sun68 Ally Solid Nylon - 40 x 1
    [quantity_1] => 1
    [amount_1] => 89
    [item_number_1] => 15731
    [bn] => WooThemes_Cart
)

After that nothing happens…no more logs, no more api calls…

Could you help me?
Thanks a lot!