I have defined a dynamic field which is stored=false, indexed=false and docValues=true. Attempting to set this field's value to null via atomic update does not remove the field from the document. However, the document's version is updated. Using atomic updates to set a value for the field does work. Tested on 6.6.2 and 7.2.1.
An example of a non-working update query:
An example of a working update query: