Most custom metadata fields are not searchable

I’m a developer trying to create a search front-end for our self-hosted Kaltura instance.

We have created and populated several custom metadata fields, and set several of them to be searchable using Settings > Custom Data > Edit Entries Metadata Schema at /index.php/kmc/kmc4#account|metadata.

But only Title, Subject, and Media Number fields are searchable in the API, or in the Content > Entries page at /index.php/kmc/kmc4#content|manage. Other field values such as Description, Keywords and Producer appear correctly in search results, but searching them gives no results.

My API calls use parameters like this (where the search field is Description): service=media&action=list&pager[pageSize]=100&pager[pageIndex]=1&filter[freeText]=&filter[advancedSearch][type]=1&filter[advancedSearch][objectType]=KalturaMetadataSearchItem&filter[advancedSearch][metadataProfileId]=4&filter[advancedSearch][items][0][objectType]=KalturaSearchCondition&filter[advancedSearch][items][0][field]=/[local-name()=‘metadata’]/[local-name()=‘Description’]&filter[advancedSearch][items][0][value]=africa

Do fields that are changed to searchable need to be re-indexed? If so, how?