Nested Coroutines > 'Parent Loop' breaks when I stop the nested routine.

I might very well be using the wrong terminology and this might all be a very wrong approach but what is happening is that I do have a coroutine state machine with a main loop using while(true) and yield return StartCoroutine. Everything concerning the looping through the current state works fine but now I have a transition loop outside of the statemachine that watches out for evens that trigger a new state at any point. To make that break happen I first store the coroutine in a variable to stop it specifically inside this transition method. The state gets changed and the state coroutine does get stopped, but the original 'parent loop' also stops. I fixed it by just restarting the loop after stopping the nested state but it seems like a dirty fix - SOOO why does this happen in the first place? ^^

 public State currentState;
 Coroutine currentCoroutine;
 
 IEnumerator FSM ()
     {
         while (true)
         {        
             yield return currentCoroutine = StartCoroutine (currentState.ToString ());
         }
     }
  
 
 void SwitchState()
 {
     StopCoroutine (currentCoroutine);
     currentState = State.Chase;
     StartCoroutine (FSM ());
 }