Model

public class MyClass
{   
   public ObjectId Id { get; set; }
 
   public bool MyBoolean { get; set; }
   public string Name { get; set; }
}

Filter with expression

var filter = Collection.Find(x => !x.MyBoolean)
// filter.ToString()

converts into

{ 
   "MyBoolean": { $ne: true}
}

but should converts into

{ "MyBoolean": false }

cause its boolean and it is not ignored (can't be `BsonNull`).

It's important in cases where you want to use sort by index and had compound index with "MyBoolean" in it.
So now if you had index like {MyBoolean: 1, Name: 1} this query can't use it:

Collection.Find(x => !x.MyBoolean).SortBy(x => x.Name)

because of filter {"MyBoolean":{$ne:true}} and you have to write it like this:

Collection.Find(x => x.MyBoolean == false).SortBy(x => x.Name)

to get filter

and achieve sorting by index.