We have recently upgraded everyone to Excel 2016.
There are a few Excel files that include pivot tables which pulls data from other excel files.
When we open the main Excel file that contains the pivot tables, Excel pulls the updated information from the other excel files.
That process works fine. However, even after Excel updates the pivot table with the external sources, the source excel files are locked and cannot be edited.
Before we moved everyone to Excel 2016 we had a mix of Excel 2010 and 2013 which did not have this issue.
I went and install Excel 2010 on a computer and when I open the same main Excel file, none of the other files are locked and can be worked on.