In OS X 10.15, all Excel documents show text upside down and backwards

If the version you have starts with 15.X you are incredibly far behind in updates. You're so far behind, AutoUpdate won't show you available updates. The 16.X series is current. Download, install and then run the latest version of AutoUpdate to update Office. https://go.microsoft.com/fwlink/p/?linkid=830196

This fixed the problem for me this morning!

Excel on the MacOS is pretty bad all around. Filtering a 100 line spreadsheet hangs for 20 seconds.

What is the logic of upgrading OS but not updating the software?

Maybe don't do anything important on your Mac if you've upgraded to Catalina...