JDK 13 release schedule was finalized today: https://mail.openjdk.java.net/pipermail/jdk-dev/2019-March/002736.html
JDK 13 is planned for GA on 2019/09/17, which would coincide with 19.0.0.9 (early Sep) if there are no changes needed from the Liberty side, otherwise 19.0.0.10 (early Oct) if we need to react with some code changes.