The fix in CDRIVER-2418 is ignores an explicit readConcern option passed to the generic mongoc_client_command_with_opts() helper, as it only processes a read concern if is_read_command is set on the command parts (not done for MONGOC_CMD_RAW).

Note: this issue only exists with mongoc_cmd_parts_assemble()'s preparation of the command for OP_MSG. _mongoc_cmd_parts_assemble_mongos() and _mongoc_cmd_parts_assemble_mongod() do not demonstrate this issue because they decide to append a read concern based purely on whether a readConcern option has been provided (and satisfies the wire protocol version) and the command is not a getMore.