A coworker of mine had a Cinema Display plugged in to the Thunderbolt 2 port on her old laptop. When she upgraded to the Thunderbolt 3 MacBook, she bought a Thunderbolt 2 -> 3 adaptor for the monitor.
Turns out that it was a mini-DisplayPort version of the Cinema Display, and that even though it worked fine with the built-in Thunderbolt 2 port, it didn't with the one in the adaptor. She ended up trading her Cinema Display with somebody who had the Thunderbolt version and an old laptop, because at the time that was easier than finding an mDP->USB-C adaptor.
Turns out that it was a mini-DisplayPort version of the Cinema Display, and that even though it worked fine with the built-in Thunderbolt 2 port, it didn't with the one in the adaptor. She ended up trading her Cinema Display with somebody who had the Thunderbolt version and an old laptop, because at the time that was easier than finding an mDP->USB-C adaptor.