Hello, I don't know whether ur data is just hiden or deleted when rolling back OS 8.1. If it's just hiden, u can find ways to open your folder and find your data. But if it's deleted, that's would be trouble. I'd suggest u to find some third-party software to help u. I know a software focus on data recovery:
easeus data recovery software. If u google it u can find their official website. and u just want to recover the game data, so u can use the free version. If no method to help u, just try it.

By the way, If u use the third party software, remember: don't put any new data to your phone, or your game data will be covered by something else.
Good Luck.