We have a 32 node cluster where each node is a 3 member replicaset having a hidden member.
Cluster is running MongoDb 3.2.5 with WiredTiger on RHEL7.
We see that some mongos processes get stuck and just do not return even for a very simple query while most other mongos processes are not affected.
This happens to different nodes and is not limited to specific machines. It had happened yesterday on another machine and then after restarting the cluster, it happened on this machine.

Queries like count() or show dbs or show collections that most probably access only the metadata - work fine on the mongos of the affected machine. However, running any simple query stalls the mongos.

Attaching mongos logs for a mongos process that is not responding as well the config server primary node log.
Note that the mongo processes were started with -v flag to increase logging verbosity