I noticed the issue when upgrading from mongoose 5.8.2 to 5.8.3 which upgrades the mongodb lib.  

More details here:

https://github.com/Automattic/mongoose/issues/8456

Here is the stack trace

{{mongodb://localhost:27018/test_db

dbOptions={"db":

,"mongos":{"poolSize":5,"socketOptions":

}}

MongoNetworkError: failed to connect to server [localhost:27018] on first connect [MongoNetworkError: connection timed out
at connectionFailureError (/opt/app/node_modules/mongodb/lib/core/connection/connect.js:377:14)
at Socket.<anonymous> (/opt/app/node_modules/mongodb/lib/core/connection/connect.js:276:16)
at Object.onceWrapper (events.js:299:28)
at Socket.emit (events.js:210:5)
at Socket.EventEmitter.emit (domain.js:476:20)
at Socket._onTimeout (net.js:468:8)
at listOnTimeout (internal/timers.js:531:17)
at processTimers (internal/timers.js:475:7) {
name: 'MongoNetworkError',
[Symbol(mongoErrorContextSymbol)]: {}
}]
at Pool.<anonymous> (/opt/app/node_modules/mongodb/lib/core/topologies/server.js:433:11)
at Pool.emit (events.js:210:5)
at Pool.EventEmitter.emit (domain.js:476:20)
at /opt/app/node_modules/mongodb/lib/core/connection/pool.js:577:14
at /opt/app/node_modules/mongodb/lib/core/connection/pool.js:1007:11
at /opt/app/node_modules/mongodb/lib/core/connection/connect.js:31:7
at callback (/opt/app/node_modules/mongodb/lib/core/connection/connect.js:247:5)
at Socket.<anonymous> (/opt/app/node_modules/mongodb/lib/core/connection/connect.js:276:7)
at Object.onceWrapper (events.js:299:28)
at Socket.emit (events.js:210:5)
at Socket.EventEmitter.emit (domain.js:476:20)
at Socket._onTimeout (net.js:468:8)
at listOnTimeout (internal/timers.js:531:17)
at processTimers (internal/timers.js:475:7) {
name: 'MongoNetworkError',
[Symbol(mongoErrorContextSymbol)]: {}
}}}