= a!forEach( pv!documentos, a!update( fv!item, 'recordType!{GC Documento .fields.{389973d7-a3f2-4ce3-aafb-9c2021e72ce9}propostaId', pv!idProposta ) )
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This thread might be helpful: https://community.appian.com/discussions/f/data/30937/the-input-records-must-refer-to-record-types-with-data-sync-enabled-that-use-a-database-as-the-source-received-the-record-type-name-which-is-related-to-the-the-record-given-in-input?pifragment-13604=2
It could be related to the version of Appian you're running and if you related record data type is from a different source.
Alternatively, did you try using a constructor instead of an update statement?
i.e.