The only way for me to get the last relic is to wait for a bounty in the Metro Station. Once I get the bounty and try and go through the door where the last relic i need is (Relic: Plymouth Rock Fragment), the door is open, but you cannot go through it. Parts of the door are actually floating in the air. Anyone else have this issue?
I go down the main escalator and I fight the first mobs (during the bounty) There are only 2 escalators I see. The one straight in front of me...its gated. The one to my immediate left is gated, but it lifts up after I defeat the first mob so I can do down and kill the bounty. From there, you go down the left escalator and thats where the bugged door is. I dont see another path down.
Just in case anyone comes to this thread looking for a way to get the Plymouth Rock Fragment, I found a way to do it after you've completed the side mission.
What you'll need: ~2 bounties. If there are only one, or none, currently available in Judiciary Square, look for a Public Execution event in that zone and save both hostages. You'll be rewarded with a bounty for that zone. If you need two bounties from this method, you can abort the bounty from the map and it'll stop the countdown, but save it to complete later.
Once you have two bounties, start one up. The first one will probably take you to the DCPD headquarters or, if you're really lucky, the Metro station. If the bounty takes you to DCPD, complete it. Then, while you're still in the DCPD building, fire up your other bounty. Since you're in DCPD, it'll have to send you to another bounty location in Judiciary Square. I was lucky that the 2nd bounty I started took me to the metro and I was able to pick up the artifact.