Here is the interesting thing regarding Display Port: I am using an OWC Thunderbolt 2 Dock. I have my dock hooked up to using the Apple Thunderbolt 3 to 2 adapt

Here is the interesting thing regarding Display Port: I am using an OWC Thunderbolt 2 Dock. I have my dock hooked up to using the Apple Thunderbolt 3 to 2 adapter. I am able to use my external 27" just fine. I have a DisplayPort cable plugged into the second Thunderbolt port on the dock. It's all working. This is reminiscent of the way the Thunderbolt Display behaves: You cannot plug a DisplayPort monitor directly into the Thunderbolt Display, but you can daisy-chain another Thunderbolt device to the Thunderbolt Display, and then plug a DisplayPort cable into the Thunderbolt device on the end of the chain, and then it works.