SharePoint access from third party application

Hi Brad, perhaps you can help me even if this post is very old.
We have a third party application that access to our Sharepoint lits.
The code has not change in the past 4 month but til 1 month the code is not able anymore to get the lists content:
We can access to the site, the array of lists definition but when we try to get the list content is always empty (even with a simple query like get the first 10 items).
When I perform the query, is not returned any error.
I post my code below:

try
{
clientContext = new PnP.Framework.AuthenticationManager().GetACSAppOnlyContext(siteUrl, clientId, clientSecret, PnP.Framework.AzureEnvironment.Production);

clientContext.Load(clientContext.Web, p => p.Title);
clientContext.ExecuteQuery();
Console.WriteLine($"Title: {this.clientContext.Web.Title}"); //ok

ListCollection lists = clientContext.Web.Lists;
clientContext.Load(lists);
clientContext.ExecuteQuery();

foreach (List list in lists)
{
Console.WriteLine($"- {list.Title}"); //ok
}

List oList = this.clientContext.Web.Lists.GetByTitle("RichiestaAssenza");
List test = (from list in lists where list.Title == "RichiestaAssenza" select list).FirstOrDefault(); //ok
CamlQuery query = CamlQuery.CreateAllItemsQuery(10);

ListItemCollection listItemCollection = oList.GetItems(query);
this.clientContext.Load(listItemCollection);
clientContext.ExecuteQuery();
Console.WriteLine($"- {listItemCollection.Count}"); //always zero

}
catch (Exception ex)
{
throw;
}


Do you think that there is a good way to understand which could be the problem?