My friend just went in to fight the Omen of Silence in End of Ruin just before getting to the part where you pick your mastery, and he died in loading screen because the loading screen either was longer than the grace period or because that area for some reason is bugged in regards to grace period.
You need to make the grace period start once a player has finished entering the portal (out of loading screen) client-side, instead of starting it once the player has entered the portal server-side.