In Playback mode you can hammer away on the "Show Route" button but it does nothing yet it works perfectly for the same aircraft in Live View.

e.g. https://www.flightradar24.com/2018-1.../H25B/1ed7cea4

Originated at Billund but you have to click-and-scroll to follow the flight track back to determine that. You can pick any of the other aircraft and see the same problem.