I was finally able to reproduce the problem that user Project Templates do not get updated, as this thread tells. In certain situations they can still appear (likely under the “more” tab) in the Steinberg Hub, but the related folder in the MediaBay stays outdated. This will be fixed in the next Cubase / Nuendo update.
Rescanning in the MediaBay as proposed here, is the recommended workaround until the update is released.
(Note that all factory Project Templates were updated with C14 and currently do not contain any of the “Recording” category. That is a different topic.)