In my last playthrough (which was bricked by a bug) the first 4 boss dungeons I found had End related mobs (3 Endermen and 1 Shulker), all of which dropped Netherite gear. My current playthrough, the first boss dungeon spawned a Shulker that dropped Netherite gear. All of this was in the Overworld before going to the Nether.

Do you know what version the dungeons were generated on? They lock in a boss at gen time, so they may have predated the dimensional bosses

5.6.1 for both worlds.

Not seeing this occur anymore, going to mark it as fixed. Update if it continues to occur in new chunks with 5.7.0