I have such field in Document

[BsonDictionaryOptions(DictionaryRepresentation.ArrayOfArrays)]
public Dictionary<LinkAt<AdvertType>, decimal> AdvertisementTypeAndPrice { get; set; }

LinkAt is Document too

now this field is not null, exactly it has 3 keyvalue pairs

but when i try to do this code
acc.AudienceGroupPercentage = null;
acc.Save();

it doesnt saves. No exeption, but if i am trying to get this object another one time, i get the start value with 3 keyvalue pairs.

Found this issue in google, but it is not related to C# and mongo driver exactly, but may be helpfull
https://github.com/doctrine/mongodb-odm/issues/1671