Go to Settings -> Edit Current Profile -> Tabs -> Tab Title Format and fill in "%w : %u" and hit apply. If you go and edit it again, you will see that it does not save it at all. Reproducible: Always
I followed the steps you listed and the tab title format settings appear to persist for me. Application Version: 18.08.2 KDE Plasma Version: 5.14.1 KDE Frameworks Version: 5.51.0 Qt Version: 5.11.2