I bled the brakes yesterday and today I noticed brake fluid is leaking at the back of the right front wheel.

The wheel cylinder and flexible line are new.
The flexible line was installed at the cylinder first, using the copper seal washer.
The bleed nipple was tightened after bleeding.

I can't tell how/where it's leaking.

My first choice is a new copper seal...

What are the likely causes?

The front brake cylinders shipped with one copper seal each.
It's a copper ring that can fit on threads at either end of the flexible line.
I put this at the connection between the cylinder and the flex line...

Was that incorrect?