The subtype of a BinData object can change when the database is backed up and restored using MMS.

In particular, a source database which contains a BinData object with 'subtype 2' will be converted to a BinData object with 'subtype 0'. While the data is still intact this is a change in format which may not cause some confusion ("why does not restored database not contain the same data as the source database?" - it does at the logical level but not at the physical level)

Subtype 2 is deprecated as per the 'BSON spec' but we do not document that the backup and restore process will automatically convert the subtype.