I have iBird Pro on iOS (iPhone 8, iOS 14.2). Bought the Photo Sleuth add-on months ago. A few weeks back I noticed it had disappeared. Hit "restore purchases", it started the download, and got to the "final stage of installing" message, where it stayed for a minute or two and then the application crashed. Add-on was not installed. Tried again, and it didn't crash, but instead just sat there on the "final stage" message for 20+ minutes before I gave up.
I tried offloading and reinstalling iBird Pro as well; didn't fix the issue.