There's a known issue with Google Play, where an app suddenly appears under the wrong Google account on Google Play, which causes drupe not to recognize the purchase.
Do you have more than one Google account synced to your device? If so, it might be that the purchase record is on another account.
Here is a way to move the app back to the right account:
- Uninstall drupe from your device.
- Open the Play Store app.
- Make sure you're using the right account by tapping the Menu icon (top left corner) and checking the email address.
- If the account is not the right one, change it by tapping the small arrow next to the email address and selecting the right account.
- Open device settings -> Apps -> Play Store -> Storage & cache -> Clear cache.
- Search for drupe in the Google Play store.
- Download and install drupe.