We are using the NEST library with Elasticsearch versions 7.14.
Lets say we have assigned a field a value of "hello World".
The filed is mapped as a Keyword type and Not Indexed.
If you update that document using the Update API with the same field having a null value, Elasticsearch ignores that assignement and keeps what was originally there i.e. "Hello World". In order to get rid of the fields value you have to assign it something like an empty string which classes in .NET with string properties normally have a default value of NULL.
So when a user updates a field by removing its value in the UI, the index does not take that change and keeps the old value.
So you actually have a completely different data in the index than what you think you have after an update.