I'm not sure how long this has been an issue, but a co-worker pointed out with 17.0 (confirmed in 17.1 beta) that if you Export Dataset on a data grid and choose "Excel File" as the format and then remove the suggested ".xlsx" extension from the file name, TOAD will default to ".xls". This causes Excel to open the resulting file in Compatibility Mode. Could the default file extension be updated to ".xlsx" from ".xls" if removed from the file name by the user?
I've logged this so I don't forget about it. I agree. The default should be xlsx. I think normally it is, but maybe some condition can cause that to change.