I have followed this tutorial:
Updating ArcGIS Online Feature Services using UPSERT (safe.com)
As the last screenshot shows, I have 1495 records.
Comparing the original CSV used to do the initial data load and the updated CSV, the new records in the updated CSV have not been added and existing records with changed attributes have not been updated.
The tutorial suggests this is possible using the UPSERT operation.
Example update I expected with the highlighted text the changed attribute value:
1000,ZYX3116631180,-123.093432,49.281221,31180,-53.7761,-21.7304,1,2011-12-02 22:01
Example add I expected:
9000,ABC1069610710,-123.093414,49.281637,10710,-79.377,-17.3218,1,2021-12-06 1:41