Considering that 5.12 is an LTS release, they are evaluating to separate WebEngine from the normal QT release cycle.
> This is still under research, but there seem to be quite tangible benefits from carving out Qt WebEngine to a separate item. Current thinking it that as a separate item we should be able to release Qt WebEngine every three months, following nicely the schedule of Chromium releases every six weeks. For Qt 5.12 LTS users this would mean the possibility to get not only the security updates, but a full new Chromium version supported on top of the long-term-supported release of Qt.