There should be a warning before fighting the final boss. Once you beat them you can no longer finish up any named hunts, turn in the last tome (which is in an area beyond the final boss), or access the secret location.

I had a few high tier named hunts left that I was struggling with and found the final boss easier than them. So I thought I'd have a chance to beat them after the final boss but before beginning NG . But no. As soonn as you beat the final boss, you are forced to choose an ending (locked into the zone) and then you automatically begin NG afterwards. There are some suggestions to create a second save on PC but that isn't an option for console.

Very frustrating that I now have to play a new character just to do the things I was locked out of.