Hi Leandra,
This is a known issue that other developers have run into recently — Shop metafields now require a metafield definition to be accessible via the Customer Accounts API (and the Storefront API). Unnamed/definition-less metafields will return null on those surfaces, even though they still work fine through the Admin API.
The fix is to create a metafield definition for each Shop metafield your Wishlist extension needs to read. You can do this via the Admin GraphQL API using the metafieldDefinitionCreate mutation, or through the Shopify Admin under Settings > Custom data > Shop.
Once the definitions are in place, the Customer Accounts API should start returning the values as expected.
Another developer confirmed this same resolution here: https://community.shopify.dev/t/without-definition-shop-meta-field-get-from-the-customer-account-extension-return-null/34623
Hope that gets your Wishlist Page back up and running!