Until Opera 60.0.3255.27 the behavior was as expected: You "minimize" Opera with the red button and it will be (active) in the dock. Click the Opera-Icon in the dock again and it will open instantly a new window. That's what I want. With the latest Opera ..... .65 a click on the (active) Opera-Icon in the dock causes... nothing. The Opera app appears in the menu bar without a new window, you have to click inconvenient "Open a new window" or "Open a new tab" in the (context)- menu.
It works properly for me. Just a thought, though: do you have multiple desktops? If so, could opera be opening the window on a different desktop from the current one?
I notice, for instance that if I have an app in full-screen—which is my normal way of working with Opera—MacOS creates a new temporary desktop for that. If I then go to the desktop on which I launched Opera, the opera menu-bar is still there, but there is no Opera window on it. Could something like that be happening to you? By the way, I have Opera set to launch on Desktop 2 out of three.
EDIT: As you were. I've just discovered I wasn't fully updated. Now I'm on 60.0.3255.56, I'm seeing the same as you. I have to say, it doesn't impact me on the way I work, but yes, let's hope the devs do something about it!