"Application.Quit" leaves Excel running in the background

I have a small excel file that is launched by a scheduling app every 15 minutes.

Functions in the excel cells read data from various places on the network and stores it in cells in this excel file. That all works perfectly.

VBA code then saves the file and does an Application.Quit.

In the previous version of excel, it worked great. Since upgrading to 2016, Application.Quit closes the "excel interface" but Task manager shows a hundred "Excel.exe"'s still sitting there using up 40MB of memory each.

The attached picture shows only five excel.exe's in Task Manager since the computer had only been running less than two hours at this point. But after 24 hours there are about 100 of them. So the computer crashes within a few days.

Thanks for the speedy assistance. I'm not sure if I'm supposed to put my responses here by editing my original question.

Here is the code. The actual updating of cell values is performed in the cells themselves. There is an Add In that reads values from some PLCs in our factory. That all works fine.

Private Sub Workbook_Open()
    Application.CalculateFull
    ActiveWorkbook.Save
    Application.Quit
End Sub

When the excel file it launched, it updates and closes. This has worked for many years until we upgraded to 2016. Application.Quit would make excel go away completely. Not now.