Reading VBA property set with C# within VBA workbook event

I have a C# VSTO / VBA hybrid project.

When a user opens a valid Excel workbook using my VSTO app, a VBA property is set from C# as such:

worksheet.GetType().InvokeMember("AddInIsLoaded",
                                             System.Reflection.BindingFlags.Default |
                                             System.Reflection.BindingFlags.SetProperty, null, worksheet, new object[]{true});

This works as expected. There are a then a few other processes within the VBA, and at each stage I check if the property is still set using

MsgBox AddInIsLoaded

Each time I receive the affirmative message.

When the user selects 'SaveAs', if the AddInIsLoaded property is true then I want control of the save functionality to be handled by my VSTO app. The issue is that I have an event handler as such

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)

If SheetExists("Protocol") = True Then
    If AddInIsLoaded = True Then
    Exit Sub
    End If
End If

End Sub

Within this event handler the AddInIsLoaded property always shows as false, even though right up to when I press SaveAs, the property shows as true.

Does anybody know any reason why this would be the case?

Thanks.