CDRIVER-4034 included a workaround to explicitly set causalConsistency to false if snapshot was true when constructing a session entity in a unified test. This could potentially override an explicit value for causalConsistency if a test were to specify both as true.
Note: no such test exists because it would require expecting a failure when constructing the session entity (before any test operations are run). This could only be done through a valid-fail test in the unified test format spec, which currently isn't use for testing other specifications. In this case, the conflict between both session options is covered by a prose test.
Having said that, the Causal Consistency spec was changed as part of Snapshot Sessions to not default to true if snapshot was explicitly set to true. Fixing this should entail changes to the session options API and entail reverting the above work-around in the test runner.