Apache Calcite Fails to Parse Its Own Google BigQuery Output - google-bigquery

Question

I'm new to Apache Calcite and am running into a strange "gap" - given a simple select query:
select * from orders
I parse it using:
SqlParser.Config sqlParserConfig = SqlParser
.configBuilder()
.setConformance(SqlConformanceEnum.LENIENT)
.build();
SqlParser parser = SqlParser.create(sqlQuery, sqlParserConfig);
SqlNode parseAst = parser.parseQuery();
CalciteCatalogReader catalogReader = buildCatalogReader(schema, typeFactory);
SqlValidator.Config validatorConf = SqlValidator.Config.DEFAULT.withSqlConformance(SqlConformanceEnum.BIG_QUERY);
validatorConf = validatorConf.withIdentifierExpansion(true);
SqlValidator validator = SqlValidatorUtil.newValidator(SqlStdOperatorTable.instance(),
catalogReader, typeFactory,
validatorConf)
validator.validate(parseAst).toSqlString(BigQuerySqlDialect.DEFAULT).toString();
Which results in:
SELECT ORDERS.o_orderkey, ORDERS.o_custkey, ORDERS.o_orderstatus, ORDERS.o_totalprice, ORDERS.`o_order date`, ORDERS.o_orderpriority, ORDERS.o_clerk, ORDERS.o_shippriority, ORDERS.o_comment
FROM ORDERS AS ORDERS
(reasonable, note the quoted identifier for `o_order date`, necessary due to whitespace in columnname)
If I then take that query string and pass it back through (setting conformance to SqlConformanceEnum.BIG_QUERY) the parse fails with:
org.apache.calcite.sql.parser.SqlParseException: Lexical error at line 1, column 95. Encountered: "`" (96), after : ""
Puzzling on face, I tried again with a parse config:
SqlParser.Config sqlParserConfig = SqlParser
.configBuilder()
.setConformance(getConformance(SqlConformanceEnum.BIG_QUERY))
.setQuoting(Quoting.BACK_TICK_BACKSLASH)
.build();
to force handling backtick-quoted identifiers and I up with:
SELECT ORDERS.o_orderkey AS O_ORDERKEY, ORDERS.o_custkey AS O_CUSTKEY, ORDERS.o_orderstatus AS O_ORDERSTATUS, ORDERS.o_totalprice AS O_TOTALPRICE, ORDERS.`o_order date`, ORDERS.o_orderpriority AS O_ORDERPRIORITY, ORDERS.o_clerk AS O_CLERK, ORDERS.o_shippriority AS O_SHIPPRIORITY, ORDERS.o_comment AS O_COMMENT
FROM ORDERS AS ORDERS
which is usable... but
why is the "default" conformance for BigQuery setting DOUBLE_QUOTE when bigquery uses backticks
why does loop-parsing lead to a different query than on the input? (running the aliased query back through a second time gets itself back, so it does stabilize, but the initial inconsistency is weird