When you open SQL Editor, then change the Current Schema dropdown, enter a select statement (e.g. select * from tablename) and run it with F9 (single statement) it selects the data from the relevant schema. When you press F5 (Run as script) with exactly the same settings, it does not run (Table or View does not exist error). It will work if you use the schema name followed by table name. e.g. select * from scott.tablename.
This is the intended behaviour because the nature of a script is to perform every operation without user intervention