Yes, when the leader dies the communication is lost.
You can read about it here RAFT - Design concepts
You can try best-effort queries. https://dgraph.io/docs/clients/raw-http/#running-best-effort-queries
If this scenario happens many times you should analyze the context and find the reason behind the leader fall.
Mutation, especially they, need to request/lease UIDs, timestamps and commit the transaction process. Without the leader this is compromised. Followers just forward the transaction to the leader.
I also recommend that you put the address of all Zero servers in all your Alphas. So the Alpha instance itself will communicate directly with another Zero to quickly identify who is the leader.