Why did I see the username of someone I follow below an ad on Instagram?
Ads you see on Instagram may show likes from accounts you follow. For example, you might see the username of someone you follow below a post or ad they’ve liked. Similarly, your followers might see posts you liked that relate to the ads they're seeing. These posts will only be shown to your followers and will adhere to any privacy settings you've set for your Instagram account, as well as any ad settings associated with your linked Facebook account, if you have one.
To see your Facebook Ad settings, go to your Ad Preferences.