Saved import mapping doesn't work for multi-record custom sets

When importing a multi-record custom field set, the mapping saves correctly. Tables civicrm_mapping and civicrm_mapping_field appear to be correct. However, when you go to use the saved mapping, only the first (top) field appears to have been saved - in my case the first field was the external identifier. The import then fails with 'unknown error'. Even so, the mapping tables are OK but the import fails. Using release 5.24.4