I could be wrong but I believe this is a limitation of the Excel's "Jet" Engine, the driver that allows Tableau to connect to it. If I'm not mistaken, you have to force Tableau to use the legacy driver to get the functionality you had before.
Again, I could be wrong... Try these resources below. Pay close attention to step two on the first link.
Your lead got me heading in the right direction!
While it still doesn't allow me to do a Fulll Join for some reason, I am now able to at least create a custom SQL using UNION to join the 2 tables.