AsynchPostBackTrigger still refreshes the whole page at any other time

I just implemented a small one UpdatePanel

with a button outside of the panel that changes the controls within it. In the instructions here and here, I have set a trigger to Page_Load

that looks like this ...

UpdatePanel1.Triggers.Add(item: new AsyncPostBackTrigger
{
    ControlID = Button1.UniqueID
});

      

        
        
        
      

    

... to allow the button click event to update the panel asynchronously (it updated the whole page before). However, it now runs asynch for the first time, but every other click after that triggers an entire page refresh. I am very confused about this. Can anyone determine what happened? ( Edit: For clarification, below are the results of refreshing a series of clicks starting from page load: Asynch (good), Whole Page (bad), Asynch, Whole Page, Asynch, Whole Page, etc ...)

The FYI form is ASP.NET 4.0 and is in a SharePoint 2013 Visual Web Part if that matters.