ElastiCache will occasionally replace nodes during a configurable maintenance window ("These replacements are needed to apply mandatory software updates to your underlying host"), they refused to provide information on the maintenance failover process, but it looks similar to CLUSTER FAILOVER (source).

For ElastiCache redisclusters that do not have replica nodes for failover, this can trigger an edge case, such that rediscluster tries to free a connection referencing a local variable before it is assigned (commit). This was patched in v2.1.3.

In our case, the rediscluster client entered a bad state, and all requests to the redis node failed. The only way to recover was a manual service restart.