If a stepdown occurs during an isMaster command, the server can return an {ok: 1, ismaster: false} reponse if there is enough time before its connection is cut, since isMaster defaults to slaveOk: true. Tests running in the retryable_writes_jscore_stepdown_passthrough expect to be running against primary nodes only, so this can fail assertions.

Only two tests in jstests/core assert on this field, ismaster.js and dbadmin.js, and neither provides much value for testing retryable writes across stepdowns, so the simplest way around this is to just blacklist these tests.