This is an intermittent & infrequent issue. It happens when a cluster is starting up, only when one region server is available for region assignment temporally and master has a transient issue to talk to the only RS
When the RPC layer has a hiccup to talk to a RS, the RS will be put in failed server list and it will stay in failed server list for 2 sec(default value).
While the 20 region retires completes much shorter than 2 secs, the end result is a failed assignment.
Below are the logging for the total time spent on 20 assignment retires, it only took about 36 ms for all the retries.