Maybe he's got better since he's been away? I know GG has been over to Germany to watch him and catch up with him while he's been there so if he (and RDZ) are happy to get him into the squad next season – and Deniz' personal issues are resolved to the point of him being willing/able to come back...
I think it was more he was allowed to return for 'personal reasons', having had some confirmed issues last season. Nothing about whether he came back or not.
Maybe they are not an issue anymore? Who knows? None of our business, anyway.
I'm basing this on not a great deal, but I get the impression there was more to him moving back to Germany than just football. There were some personal things going on, I seem to remember, so it MIGHT have been the case that he asked to leave to be closer to home? Therefore, we could want him...