Decorations or not, actors disappear if they're not relevant network wise, and in the network the actor center is the one which counts, and the moment it's not visible it becomes irrelevant so it ends up disappearing (most of the time, there are other factors at place of course). Although in case of decorations, they're generally bStatic=True, so they shouldn't have any problems whatsoever.
But then there's another thing that makes them vanish, and it's a hardcoded optimization from the engine itself, which has to do with the pivot of the mesh itself, rather than the actor.
When the pivot is not in your view, there's a point when the engine "thinks" that the mesh doesn't need to be rendered anymore, and it disappears.
It's not exactly when the pivot is not in your view however, there is some leeway in terms of the angle, but it seems a hardcoded leeway that doesn't have the actual bounding box of the mesh in consideration, so you only notice it with bigger meshes.
However there are a couple of ways to avoid this to happen:
1 - You update both the PrePivot and Location of the actor in each tick and in such a way that the pivot is always in front of you while the the mesh "looks" to be at the same exact place.
The only problem of this is that the lighting of a mesh doesn't take into consideration where the mesh is, but rather where the actor itself is, so unless the mesh is bUnlit=True, it will look weird in places where there's a good lighting contrast and color.
2 - Draw them explicitly in the canvas in some way: they could be hidden, but iterated over to be explicitly drawn.
But then, the responsibility of doing optimizations like not rendering when they're out from your frustum projection or when they are in other areas and behind BSP, would fall onto you to do them.
In the case of Unreal 227, actors actually have a setting to simply disable this pivot optimization of the engine.