C# app using PDFCreator leaving PDFCreator instances running on server

I have a C# app that references PDFCreator (1.7.3). It is leaving sessions active after it completes. 

We have a large job system that converts Excel files to PDF.  Each job opens its own instance of PDFCreator using the C# app.  When the PDF file is created we would expect (it used to do this) the instance to close. as that is wha tthe C# app is telling it to do. However, this isn't happening and I have to periodically go out and clean off dozens of instances from the print spooler. 

PDFCreator is the default printer, I've modified the setting as we used to have them, so that no animations or prompts should appear.