Do you own this apartment?

Looks like a fire rated steel frame. Common in multi floor apartment buildings.

I'm trying to see that as a compression valve.....lol Usually I see a few threads under the nut.

It's got to be tightened absolutely to the top of the threads, so maybe the copper isn't pushed through the brass ferrule? That could be causing the leak.

It almost looks like it has a cpvc cconnector on the bottom of the nut.