Concatenating many FirstAsync will block completion of parents observables?

I'm working with Reactive and I was testing concatenation. Unfortunately I wasn't able to understand something. Can anyone explain why only last onCompleted: is being invoked? I'm really confused about it. Aren't all FirstAsync supposed to complete once they emit First element?
Code Example:
```
Observable
.FirstAsync()
.Do(_ =>
{

}, onCompleted: () =>
{
//Not Invoked
})
.FirstAsync()
.Do(_ =>
{

}, onCompleted: () =>
{
//Properly Invoked
});
```