UPDATE: After the most recent updates to ArcGIS Online (June 2022) it seems the below solution doesn't work. I've tried a few variations but none of them result in the split policy either being inserted or changed. Back to not having an option but to use ArcMap to republish the service as suggested here: FAQ: Is it possible to add or edit domains after a feature service is published in ArcGIS Online? (e...
Posted an Idea here to give us something that works! Update "Alter Domain (Data Management)" tool to up... - Esri Community
We've finally got a solution to this issue (with assistance from ESRI Support). The JSON at the service URL for the hosted feature layer needs to be updated to either include or modify the split policy so that it reflects the desired behaviour ("Duplicate").
My workflow below:
1. Go to item page > click on sub-layer > open Service URL
2. Modify the URL to see the Admin settings
3. Scroll to the bottom of the page and select the "Update Definition" option
4. Clear the lastEditDate value (replace the value with "")
5. Scroll down to the fields list
6. Update any field domains where domain is NOT null with the below line of text:
"splitPolicy" : "esriSPTDuplicate",
Updated JSON below:
7. Click the "Update Layer Definition" button and whalla! Your features should now duplicate attributes in fields with coded domain values when split in ArcGIS Pro 2.9+
Lindsay Raabe
GIS Officer
Forest Products Commission WA