They're meant to despawn. Head to the taken orb and shoot the blight above it. Do the plate and you'll be teleported, the actual enemies for it now spawn and you clear them out for the fragment.

This is a known bug on the Barrow-Dyad Exotic Quest—the enemies in the K1 Revelation Lost Sector sometimes don't spawn after interacting with the plate/puzzle, leaving you stuck on the quest step. What works for many players: Join another player’s session who’s at that quest step. Enemies tend to appear when the host is actively doing that part. Reload the Lost Sector several times, then enter and interact with the plate slowly—waiting a few seconds before stepping off. Some report that entering on non-max difficulty (like Heroic) instead of Legend helps force the spawn.