I was over 200k prestige and got robbed repeatedly when sending resources to MY OWN CITIES.

Will say though, that that was 3 or 4 weeks ago and things may have changed since then. It started happening when I split my cities greater distances apart. So distance between cities may be a factor as well?

I actually don't think it's a bad thing in the game, it should happen more frequently and randomly really, if it's going to be in the game then no one should be totally immune. But if it is meant to dampen resource sharing or "tribute" it doesn't really do that because no one is stopping sending resources because of it.