Thanks for the clear picture of your environment and problem.
There is some wonkiness in the data dictionary regarding temporary tablespaces in pluggable databases in 18c and 19c. I've worked around it in Toad 13.2. Please give it a try there and let me know if it's working correctly.
You can get 13.2 here. Get the same bitness (32/64) as 13.1. Don't uninstall 13.1.