The issue with ArcGIS Server not accepting requests to update feature records with JSON matching a subset of a feature record's schema is addressed in the 10.6 release.
ArcGIS GeoEvent Server 10.8.1 deployment is verified to be able to map event records to a schema matching a subset of a feature record's schema and use data it has received to add or update feature records with only attribute values and later update the feature record's geometry. A consistent TRACK_ID is required when exercising the 'partial feature record update' capability.