This is fundamentally working as designed - the assets in your Brand Library are there to support generation of Creatives and Templates.
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​Image generation is primarily achieved through text-based prompting and there is no automatic link to your Brand Library. The only link to the Brand Library is the ability to manually select image assets from your Brand Library's 'Style references' from within the Image Generation tool to influence the work of the AI model.

Screenshots below show the Brand Library's 'Style references' tab and, following that, the 'Styles' section of the Image Generation tool: