I am using the following generic code to replace a document with an updated version

 public async Task UpdateAsync<T>(T model, string collectionName) {
 var filter = Builders<T>.Filter.Eq("_id", model.Id);
 await database.GetCollection<T>(collectionName).ReplaceOneAsync(filter, model);
}

The documents are stored with a _t discriminator like that:

{ "_id" : "5c7fb949d0c890000179bb46", "_t" : "Type",  ....

After using ReplaceOneAsync the _t discriminator is removed from the file in the database.
I guess a workaround would be to manually cast model as the specific type and update all fields, which would result in _t not getting modified. (The question is if that is less performing.)