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Similarly if you created your feature service by copying the item in AGOL or using ago-assistant to copy the service, it may appear that the defaults were copied, but you are not seeing them in MobileMap or InventoryManager when you create data. Try publishing for a fresh ArcPro project as recommended above.

Other solutions if the above does not work: https://community.esri.com/t5/arcgis-field-maps-questions/default-values-not-working-in-field-maps-mobile/td-p/1116905

“… BUG-000144318 - When you publish a layer from ArcGIS Pro to ArcGIS Online that has default values set on at least one field and there are existing features the default values will not show up in ArcGIS Field Maps web app Smart Form Templates.

They[Esri] informed me of the following workaround:
You can export a FGDB in ArcGIS Online from the hosted feature layer that has these default values setup and then publish a new hosted feature layer copy from this FGDB. This copy will not exhibit the behavior. Note that this will be a distinct layer and will not have a service definition. Meaning that you can't overwrite it from ArcGIS Pro like the original layer.”

MobileMap Crashes When Editing Features or Saving Tree Table When Using a Settings File Copied from Other Service/Tablet

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