CSFLE prose tests require read/write concern majority when operating on the key vault. The test README has this note:

Perform all applicable operations on key vault collections (e.g. inserting an example data key, or running a find command) with readConcern/writeConcern "majority".

Some of the prose tests omit the read/write concern majority, and this results in flaky test failures.

Example: https://spruce.mongodb.com/task/cxx_driver_ubuntu1804_zseries_compile_and_test_with_shared_libs_extra_alignment_patch_77c52a749571029a09f056867fb9df65b866f222_6011c1d2d1fe072ff50e7763_21_01_27_19_41_07/logs?execution=0