A Tempo-Track that was exported from another Project won’t show its content visually once after it has been imported. The actual tempo-changes will in fact take place and are shown in the Inspector, too - we just can’t see or edit them graphically in the track any longer.
Workaround: Closing and re-opening the Project brings back the graphics.
100% reproducible with Nuendo 11.0.41 under Windows 10.