Workaround? You already said it: remove the semi-colon.
But seriously, it could be a bug in the JDBC driver. Probably best to report this via MOS.
Could you reproduce the problem?
Yes I'll probably open an SR for this problem.
Oh yes, reproduced on SQL Developer 18.4 against Oracle 12.2 (running in a Linux image under Virtual Box) with the bundled 18.3 JDBC driver, thin connection.