Uhm from my experience, you should go to the store the minute you see you open the app and it has no chats. Everything was restored instantly on my phone (Lumia 735 running denim) and chats started commiing back in. Also I think the bug in the previous version must be 'Archiving' as when you archive your chats, the dissappear from the chats list. Try going to options and then 'unarchive' that should also restore your chats.