As I wrote I had a very similar problem and I think my solution might work for you as well, as " (specifically when it walks on) an actor that was spawned by server." this was exactly issue I had, when I was walking as a client on a serverside spawned mesh.
My solution for that was that I not only had to replicate actor mesh is attached to but I halso had to replicate subobject (mesh) itself.
I think that problem then is, that mesh is attached to an replicated actor, which is why it does work at all, but engine can’t resolve a net guid as mesh itself isn’t replicated.