Enhance the feature service writer (AGOL, Portal, ArcGIS Server) to enable the user to configure whether an update operation should update the geometry or just the attributes, in line with other writers.
This is available on the writer as Update Spatial Columns see https://docs.safe.com/fme/html/FME-Form-Documentation/FME-ReadersWriters/arcgisonlinefeatures/feature-types-w.htm