It happens: an app you bought outright changes hands or changes its model, and one day the lifetime unlock is gone. Your photos are still there, behind a paywall.
First, get the photos out
Before anything else, export. Most apps still allow export from the free tier, sometimes a few at a time. Export to the Files app or back to Photos, then confirm the count. Do this before you delete or update the app.
Then ask Apple for a refund
Go to reportaproblem.apple.com, find the purchase, and explain that a lifetime purchase was revoked. Apple decides, but it is worth the two minutes.
What to check in the next app
Whether export and backup are free regardless of plan. Whether lifetime is a real one-time purchase (a non-consumable in App Store terms, which Apple always lets you restore). Whether the maker says in writing what happens when a subscription ends.
Vaultpod’s position
Lifetime is a non-consumable purchase: it cannot expire and always restores. Viewing, exporting and deleting are free on every plan, forever. It is in the terms and in the app’s pledge screen.