Brake emergency

Ima calling it a bad wheel cylinder.
But just to be sure disconnect the self-adjuster cable and apply the brake again. If the one piston is still lazy, I think that confirms it.
What I cant figure out is why, if you opened the system to replace one WC, you didnt pop them all open for inspection. I have a feeling you would have replaced the rest of them too. For what they're worth I would have,or,at least honed them and kitted them.
All the best to you