Apple ID is the same but the App Store says I can redoenload old purchases due to the Apple ID not being the same

See -> If there's an issue when you re-download apps, films or other content from Apple

If there's an issue when you re-download apps, films or other content from Apple – Apple Support (UK)

See also this Apple article: Leave Family Sharing

Leave Family Sharing - Apple Support

⤵️

What happens when you stop sharing?

• Your Apple ID is removed from the family group and you won't have access to any services shared by your family, such as an Apple Music family subscription or a shared iCloud storage plan.
• You stop sharing locations with your family members and your devices are removed from the family Find My iPhone list.
• If your family shares iTunes, Apple Books, and App Store purchases, you immediately stop sharing your purchases and lose access to the purchases made by your other family members. You keep any purchases you initiated while part of the family group. Other family members can't use content downloaded from your collection.
• Any content that your family shared with you isn't automatically removed from your device. You can purchase it again or remove it to free up space on your device. If you downloaded an app from a family member’s purchase history and made In-App Purchases, you'll need to purchase the app yourself to access your In-App Purchases.
• If your family organizer set up an Apple Cash account for you as part of Apple Cash Family, your account automatically closes and your balance transfers to the family organizer's Apple Cash account. If you're the organizer and you disband the family group, any Apple Cash Family accounts you set up for family members will close and their balances transfer to your account.
• If you have an Apple Watch that uses Family Setup, you won't be able to manage that Apple Watch.
• If you share a photo album, calendar, or reminders with family members, you won't share them anymore. If you want to keep using Family Sharing but not share certain features with your family, you can unsubscribe from them instead in the Photos, Calendars, or Reminders apps on your device or on iCloud.com.
• If you use Apple Card Family and you leave a Family Sharing group, you aren't automatically removed from the Apple Card Family account. Learn how to remove yourself or a participant from a shared Apple Card account.