Code assist converts whole statement (including table alias) to lower or upper case when using table pick list:
If writing a SQL statement in the SQL Editor, the window like this:
SELECT * FROM <known_table name> tes WHERE tes.
The cursor is located after the last dot and after waiting a short time you can select the column you want to use.
Since 9.6.1 after the selection of a column the alias for the table is changing to upper (or lower) case.
An enhancement request has been submitted for consideration in a future version of Toad for Oracle