A property can be defined as having a Bson Representation:

mb.Entity<Customer>().Property(e => e.Value).HasRepresentation(BsonType.Decimal128); 

And it can also be defined as having a ValueConverter

mb.Entity<Customer>().Property(e => e.Value).HasConversion<Decimal128>(); 

However if you use these two features together the Bson Representation is silently dropped.

mb.Entity<Customer>().Property(e => e.Value).HasConversion<Decimal128>().HasRepresentation(BsonType.Decimal128); 

Will store the vaule as a string (the default for Decimal128).

This scenario is likely uncommon but should be fixed. ValueConverterSerializer will need to be extended to support IRepresentationConfigurable and IRepresentationConverterConfigurable.