The "unacknowledged write concern returns disengaged optional" fails on zSeries with MongoDB version v3.7.2-206-gce91d96. That is an outdated 3.7 series server version, but it hasn't been updated in months due to BUILD-5496. Whatever is causing the test to fail there is likely due to a bug in that development version of the server. Disable zSeries testing for the C++ Driver now and wait for the "latest" server version to be upgraded there.