Left a repro attached.
It has been observed the following interleaving will cause any insertion on a timeseries collection to fail 

• create a db and a collection (timeseries) using mongos 1
• drop the db on mongos 2
• re-create the db and the collection on mongos 2
• insert some data using the (now stale) mongos 1 

The mongos 1 will encounter StaleDb and returned a failed insertion.

The issue occurs when both mongos and mongod are both in v8.0