Dawn of the Infinite megadungeon is currently dropping non-upgradable item level 421 loot on the 10.1.5 PTR.
Dawn of the Infinite megadungeon became available for testing this week and content creator MrGM confirmed on Twitter item drops have Item Level 421. As you can see in the image, the drop was a trinket (Mirror of Fractured Tomorrows) with no upgrade ranks.
We’re not sure if it’s intended because loot tables for the dungeon haven’t been implemented yet, but for now, the items cannot be upgraded on the PTR with the new system.
For comparison, Mythic+ Dungeons drop Item Level 402 Veteran loot that can be upgraded to Item Level 424.