Quick answer: Unreal Mass entities failing to find archetype after a config tweak? Adding/removing a fragment or tag changes the archetype hash - entities created with the old hash become orphaned.
Added a debug tag to an entity config. All entities spawned before the change are now invisible to processors that query the new tag.
Regenerate via spawner
Destroy and respawn affected entities. Mass doesn't migrate fragments at runtime; spawn-time archetype is fixed.
Use Optional fragments
Fragment marked Optional in the trait can be added at runtime. The archetype still shifts but Mass handles the migration.
Version your config
Tag the config with a version number. On level load, check version mismatch and reset entity pools before spawning.
“Mass archetypes are immutable per entity. Config changes are migrations.”
Build a dev command that dumps archetype counts per query. Mass tooling is sparse; instrumentation pays for itself the first hour.