When attempting to create and use user-defined title templates, browsing to the folder containing the titles yields an apparently empty folder. To work around this behaviour you can manually type the filename of the title and then press enter, this will load the title and allow you to enter text, completing the dialog puts the newly created title in the project bin, created as expected.
I saved a custom template title to an empty folder, clicked 'Add Template Title', and in the file dialog I can see the file present and load it. Application Version: 18.12.0 KDE Plasma Version: 5.14.4 KDE Frameworks Version: 5.53.0 Qt Version: 5.12.0