I just checked with our DBAs and was told "you cannot connect to a physical DB that is running Data Guard." Supposedly you can connect to a logical instance, but the physical instances need to remain offline copies until you fail over. We run Oracle 184.108.40.206 and have a Data Guard instance waiting as a standby.
Thanks for your comment.
Can you check with your DBAs, if a physical DB cannot be connected, why SQLPlus and Toad can still do a query?
One more question, did you mean, you can change between the phsyical and logical instance?
I've inquired, but one thing I can think of is that the manual queries may allow a read-only connection, and while one would think Tableau only requires a read-only connection, it may ask for a read-write connection in order to be able to do some operations in the database such as create temp tables for context filters.