This worked for me: https://community.esri.com/t5/arcgis-field-maps-ideas/copies-of-layers-causing-issues-for-download-of/idi-p/1071212 In Field Maps, I looked under Profile > Troubleshooting > View log which showed an error about not supporting duplicate feature layers. I went into the web map, removed the duplicate feature layer, then the offline area downloaded properly.
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