ACF saves the relationship as a serialized array of the post IDs in the database. That’s why you get an “array” text. In this case, could you please ask Yoast SEO support if they have a hook to modify the Title and the Meta Description? That way you can use the code example on this page to modify them: https://www.advancedcustomfields.com/resources/relationship/.
I hope this helps 🙂
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