You’ll want to start off with an odd number of Alphas to maintain a majority quorum. Because you started with 2 Alphas and stopped the second one, a majority of the Alpha group isn’t up so even if the new third Alpha joins the cluster, there’s no current leader to connect to. There won’t be until you start Alpha 2 again.
This is why we enforce that Zero’s --replicas flag is odd. In this case, you’ve set it to 3. That’s correct, but starting this test with two Alphas would get you stuck as you did here with Raft.
If you started with three Alphas and then killed the leader, then the two followers (the remaining majority) would detect that and elect a new leader amongst themselves.