The following should reproduce a MongoDB.Bson.BsonSerializationException on update.

// — Model:

public class Test

= ObjectId.GenerateNewId();

    public string Name { get; set; }

    public byte[] Data { get; set; }

    public BsonDocument Doc { get; set; }

    public static IMongoCollection<Test> GetCollection()
   

}

// — Code:

var t = new Test();

t.Name = "TestName";

t.Doc = new BsonDocument();

t.Doc["Name"] = "TestDocName";

await c.InsertOneAsync(t, cancellationToken: cancellationToken);

string? nullString = null;

await c.UpdateOneAsync(
    d => d.Id == t.Id,
    Builders<Test>.Update
        .Set(d => d.Data, null)
        .Set("Name", nullString)
        .Set("Doc.Name", nullString),      // <-- Source of error
    cancellationToken: cancellationToken);

MongoDB.Bson.BsonSerializationException: 'C# null values of type 'BsonValue' cannot be serialized using a serializer of type 'BsonValueSerializer'.'