Multi-AZ failover - replica still read only after failover
One thing I need to answer this question. Which endpoint you are using in the application Instance or Cluster? 

In Aurora there are two types of endpoints.
1. Instance endpoint
2. Cluster Endpoint

Instance Endpoint:

This endpoint will always points to that particular instance whether it is read or writer. It never care about the role, it'll point that particular instance. 

Clusrer Endpoint:

Actually it has 2 endpoints. One is for Writer and another one os for reader. This is the endpoint we should use in the application. Because unlike Normal RDS, Aurora's failover and MultiAZ is different. So these cluster endpoints will point bases on the role. 

Writer endpoint will always point to the writer role instance, after failover this endpoint will go to the new writer. Like the same for Read endpoint.

Answers for your three questions


What other information can I provide to clarify my configuration or issue?



  Check which endpoint you are using.



Any clues on why this replica instance stayed read only?



  If you are using instance endpoint, after the failover the application
  still refers the same instance for write. Thats why its failing.



If this is the expected behavior of read_only:TrueIfReplica, where else should I look to fix my configuration?



  Nothing, just again check which endpoint you are using.