Custom types FAIL to deserialize AFTER database copy

I am using a custom type in my collection, and register it on applicatin start up with:

All good

However when we copy the database (via studio 3T but i dont think thats the issue) and connect to it the ‘magic’ that enabled mongo to map to my custom type is gone…

I get this issue as soon as my code attempts to deserialize a document containing the custom type

Unknown discriminator value ‘BsonNull’.
at MongoDB.Bson.Serialization.BsonSerializer.LookupActualType(Type nominalType, BsonValue discriminator)