This is actually intended; we've done some work behind the scenes to make it so mobs don't instantly level and get harder when you ding. The idea being that when you ding from 104 to 105, you haven't had a chance to get 105 gear yet, so you're wearing 104 gear and now creatures have more health and do more damage.
I believe it's somewhere around 50% - and then it'll trigger when you complete a quest, hit a loading screen, or log out. ( / )
We are considering disabling relics in PvP instances (battlegrounds, skirmishes, arenas, and rated battlegrounds) due to way gear is handled in PvP in Legion. Item level will not have as much impact in PvP which throws off the balance of relic itemization. ( / )