This problem was discovered when having problems processing spreadsheets exported from Google Sheets. Through some testing, it has been found that if the count attribute isn't present on the dataValidations element within the sheet.xml then the validations cannot be found by POI. Manually inserting this attribute, and then repackaging the .xlsx file, and the validations can be found and processed successfully. Validating the xml against the schema suggests this attribute is optional. A set of test cases and files confirming and demonstrating the problem can be found at https://github.com/stuzart/poi-google-export-testcase
Thanks for the very detailed reproducer! This is fixed now on latest trunk via r1885011, we now only look at the array and ignore the "count" attribute to always handle elements even if count is not set.