Why this Kotlin Coroutine is freezing the interface?

Question

So, I had this code running inside a "onBindViewHolder" recycler's adapter method:

 launch(UI) {
            val bitmapDrawable = loadLargeBitmapDrawable()          
            imageView.setImageDrawable(bitmapDrawable)
  }

This was freezing my app for some seconds, locking my mainthread.

But then I changed to this:

launch { // <- I removed the "UI"
            val bitmapDrawable = loadLargeBitmapDrawable()  

            launch(UI) { //Launch the UI coroutine inside the other
                imageView.setImageDrawable(bitmapDrawable)
            }      

  }

Why this is happening? The purpose of coroutines are to make things async inside the same thread (UI) right? Someone can explain me why I had to run a UI coroutine inside another coroutine scope ?