Problem: Apple 27” Thunderbolt Display not working when connected to 16” Macbook Pro (2019) via the Apple Usb-C to Thunderbolt 2 adapter. The display remained dark, despite have just been disconnected from an older Macbook running Mojave.
First (simplest) solution: Connecting a stand-alone Thunderbolt 2 cable from the Macbook adapter to the BACK of the Display worked fine, the Display works fine.
But since I was pretty sure the display’s T2 cable is fine (even if it did not work connected directly to the Macbook) I then tried putting a Thunderbolt peripheral between the Macbook and the Display (using the Display’s built-in T2 cable). And that worked too, freeing up the T2 port on the back of the Display!
Second (better) solution: Display’s built-in T2 cble connected to a T2 peripheral (and OWC thunderbolt dual drive dock in this case) and then the peripheral’s other T2 port connected to the Macbook’s adapter.
If you use other T2 peripherals, solution 2 is superior since it frees up the extra T2 port on the back of the Display.