We are seeing unusual failures in the following test: Test_Unpin_For_Non_Transaction_Operation
https://github.com/mongodb/mongo-csharp-driver/blob/719731cb7548ae6c1cfc253c33384084f84fb3b1/tests/MongoDB.Driver.Tests/PinnedShardRouterTests.cs#L112

e.g.
https://evergreen.mongodb.com/task_log_raw/dot_net_driver_secure_tests__version~4.4_os~windows_64_topology~sharded_cluster_auth~auth_ssl~ssl_test_patch_cafa20474a43e7a3dbac23416b2262f59c761f29_5e7a30313627e0034e073190_20_03_24_16_07_14/0?type=T#L4344

It should be extremely rare for this test to fail as the probability of selecting the same mongos after 50 server selections is extremely low.