warnings on importing raw data exports into third-party tools

I'm working on automating an analysis of our course evaluation response data and have chosen to use an external tool (Perl and related modules, running on MacOS X Sequoia 15.5) to do so.  I'm therefore relying on CES's raw data export capability, which (as you'll know) yields one Excel 2016 (.xlsx) workbook per project/course/instructor.

However, the Perl module I'm using to read and parse the data (Spreadsheet::ParseXLSX) yields a warning (multiple times per row) every time it loads one of these files.  The warning message suggests that one or more of the cell format specifiers is being left undefined.  Moreover, this seems like an issue internal to the data files themselves, since I can eliminate the warning by opening one of them within Excel and then saving it under a new filename (this would be a simple workaround except that I'm dealing with a set of roughly 1,200 such files each academic term).

Any thoughts or guidance would be duly appreciated!