seems you got your money back
seems you are just curious why it came back and wondering should it have came back

short answer if the transaction is not confirmed after a while it just drops off the network as if it nver happened.
it appears coinbase noticed this(in green) and gave you the balance. so you are back to where you were, no loss no gain.

the main reason a tx doesnt get confirmed is that the tx fee isnt high enough. there is no fee that is a 'guarantee' and at the moment the devs are arrogantly avoiding a proper working 'priority' formulae/code to sort the issue. all they are saying is "pay more" like bitcoins limited blockspace should be treated like an auction house whre you have to declare a higher bid then others for a chance.