1. Launch Query Analyzer using an Oracle server connection.
2. Note that the current schema is SYS.
3. Do Reconnect first. (This step is necessary to reproduce the issue.)
4-a. Now switch to a different schema, e.g. CHINOOK.
4-b. Run a query for a table in the CHINOOK schema, e.g. select * from ARTIST
4-c. An error message is returned, complaining that the table does not exist, even though the table exists in the CHINOOK schema.
4-d. Notice that the schema drop-down switches to SYS.
5-a. Now switch to the CHINOOK schema again.
5-b. Run this query select SYS_CONTEXT('USERENV','CURRENT_SCHEMA') sid from dual (which retrieves the current schema context)
5-c. Notice that the schema drop-down switches to SYS and the query result also confirms that the current schema context is SYS.

This issue is reproducible starting in v14. I haven't tried v13 or any prior version.