One of our application's node was faced with some network issues, yet the node did not completely restore its operation after the issue was gone.  
It faced the increased number of errors "com.mongodb.MongoTimeoutException: Timeout waiting for a pooled item after.."

During network issues connections were closed because of com.mongodb.MongoSocketReadTimeoutException: Timeout while receiving message. After network issues were gone opening new connections were failed for only one of two read replicas with 
com.mongodb.MongoSocketException:<host_name_of_read_replica>
For second read replica connections were opened successfully.

Mongo Async driver v3.11.2

ClusterSettings: 
type=REPLICA_SET_SECONDARY

ConnectionOptions: maxPoolSize=10&minPoolSize=10&w=1&wtimeoutMS=500&j=0&connectTimeoutMS=500&socketTimeoutMS=2000&readPreference=secondaryPreferred&waitQueueMultiple=5&waitQueueTimeoutMS=2000&replicaSet=rs0

Issue:
I assume that during com.mongodb.MongoSocketException:<host_name_of_read_replica> ConcurrentPool doesn't release semaphore permit properly. As a result, application faced with errors like "com.mongodb.MongoTimeoutException: Timeout waiting for a pooled item after.."

Please see the attached stacktrace with some obfuscation for more detailes.