Drill uses the parquet-mr library to write Parquet files. The metadata signature that Drill produced in 1.2 and earlier versions of Drill is indistinguishable from older footers written by other tools (such as Pig and Hive). There was a known bug when those tools wrote metadata that caused the statistics to be incorrect. To correct this, the parquet-mr library adopted a behavior of ignoring statistics from the old form of the Parquet footer.
With 1.3, Drill upgraded to the latest version of parquet-mr and has now started ignoring these statistics as well. This ensures correct result but produces performance regressions (compared to Drill v1 and v2) when querying against partitioned Parquet files generated in Drill 1.1 and 1.2.