mongostat has a js test called stat_discover_shard.js that has been passing on 3.6 and failing on 3.7 nightlies.

Here is an example build with a failure: https://evergreen.mongodb.com/task/mongo_tools_ubuntu1404_qa_tests_unstable_b4547e48d74081c6ed3b607b9ea5e6acdb7b00cf_18_04_13_15_47_28

The symptom seems to be that the test is expecting mongostat to discover 3 shards and it only discovers 2 shards in the allowed timeframe.

mongostat does discovery by querying config.shards

I suspect that another test, stat_mixed_storage_engine.js is experiencing the same issue, as it also does a discovery test.