I did the same thing and didn't notice an immediate effect.  I did notice several turns later that the prestige in all of my cities had gone up by 2 points.  It may be worth testing this and seeing what's going on.

Hmmm, I don't believe that they are effecting city prestige specifically. Additionally, I believe that prestige is being rounded DOWN (which I hate) which might be messing up any results you might be expecting. Prestige is a bit of an annoying little resource, imo.

it certainly works when playing wiht a royality sov, but another hero with royality does not make a difference.