It's not an issue with field caching or mapping; rather, it's about maintaining the consistency of the AI Search physical structure throughout its lifetime. The definitions of these fields, except for "Retrievable," will consume more storage and significantly impact search behavior. Therefore, rebuilding your index is the only viable option.

Although you can add new fields at any time, existing field definitions are locked in for the lifetime of the index. For this reason, developers typically use the Azure portal for creating simple indexes, testing ideas, or using the Azure portal pages to look up a setting. Frequent iteration over an index design is more efficient if you follow a code-based approach so that you can rebuild the index easily.

https://learn.microsoft.com/en-us/azure/search/search-what-is-an-index#field-attributes

• Do I need to reset or delete the indexer before the new fields can populate? => Yes, rebuild it.

• Is there a way to force Azure Search to re-index only the new fields? => The new fields will follow the initial fields definitions (schema)

• Could this be due to indexer caching or an issue with field mappings? => No