Quick answer: Unreal Iris missing replication of a component added to an existing actor? Component-add events not in default replication - use FastArraySerializer for component list.
Server adds a buff component; client doesn't see it.
FastArraySerializer for components
Server's components list as fast array. Add/remove replicate.
Or use a dedicated proxy
Buff data replicates; client spawns component from data.
Audit per component type
Each replicated component: add/remove path documented.
“Component add isn't auto-replicated. Replication is opt-in.”
If you add components at runtime in multiplayer, the explicit replication is mandatory.