After upgrading Mongodb cluster and mongos to V3.2.12, we do see mongos connectoins behaviour changes. we see application read performance is impacted.

On March 20 afternoon, all mongos were upgraded to V3.2.12, mongos connections graph changed dramtically. Did the V3.2.12 mongos close connections more actively ?
How can we avoid this behaviour ?

Mongo Java Driver version is 3.3. Mongodb is V3.2.12 (MMAPv1)