Products do not get immediately updated in Facebook after the default Google Product Category (GPC) fields are updated at it's category level (Products > Categories > Add/Edit category) or at the global level (Marketing > Facebook > Product sync > Default Google product category).

Forcing a full product sync via WooCommerce > Facebook > Product sync > "Sync products" button will cause the products to be updated correctly.

If the feed sync method is working correctly and updating the whole catalog once a day we should be able to rely on that to update these kinds of changes that affect many products. Alternatively we could potentially use the Batch API to just update one field like GPC without needing to send the product.