I just want to confirm, the issue described is with the Subscriptions page for Sandbox Apple ID's created in App Store Connect. Only those accounts can sign-in to sandbox and are visible in iOS Settings app under App Store. Tap your signed in sandbox apple ID > tap 'Manage' and then tap 'Subscriptions'.
While apps installed via TestFlight, the only Subscription management experience available is via the
If you are experiencing issues access the subscriptions page with you sandbox Apple ID, please provide detailed steps, screen recording and a sysdiagnose - then reply with the FB# and we will review asap.