After closing a cultural treaty deal with other CIVs the effects are not visible on the planetary culture value breakdown - dos not seem to have any effect even if the treaty is concluded with several CIVs
It also seems to be a potential balancing issue how this bonus would be applied as according to the description it gives a 25% bonus but this would not take into consideration how powerful the individual treaty members are. Suggest the bonus is applied as a +percentage of the other CIVs overall culture thus less powerful CIVs give smaller bonus
Update: Economic redistribution UN treaty also does not seem to be applied at least when the player has the largest treasury (no additional costs displayed on economic screen)