In 184.108.40.206 (and before), altering a user through the Alter User screen always sets the user’s default tablespaces to the first ones in the lists, even if the tablespace tab/screen was not visited.
In the SB, I select a user from the LHS user tab and then the “Alter User” button on the LHS. Change something simple like the “Account is Locked” checkbox and hit the “Show SQL” button. Toad wants to change the tablespaces as well. The tablespaces in my case appear to be alphabetically first in the dropdown list on the tablespaces tab of the Alter User screen, even though I didn’t go into that one.
The workaround is to grab the SQL from the “Show SQL”, drop it into the Editor and remove the tablespace change.
This works correctly in 220.127.116.11 GA.