In Adobe Commerce, to retrieve custom extension attributes on an entity such as a quote, you need to ensure that these attributes are properly loaded when accessing the entity directly via repository interfaces. When fetching a single quote, it is essential to use a plugin on CartRepositoryInterface::get to include the extension attributes as this method is responsible for loading individual quote data.

If the plugin is missing for the get method, the extension attributes won’t be loaded for single quote retrieval, leading to their absence in the result.

Additional Resources:

Adobe Commerce Developer Guide: Extension Attributes

Magento 2 Repositories and Plugins