When me Xperia z5 screen blocks by itself and last that way for a few minutes, When you try to unblock it, it turns green in some sports and you can notice the fading because is too slow. Once the fade is gone, all is normal.
Hard to tell what's the issue, you could record a video > upload it to youtube > post the link here or you could contact your Local support to get it inspected.
It's hard to tell what this issue is based on the description. It could be anything really. Does the same thing happen in safe mode? If so, can you please try a software repair and see if it's any better?
