I loaded jacyp's save game and confirm his description of what happens. When you mouseover the Liberate the City option, the tooltip states:
"This City was formerly owned by Darius I. You have the opportunity to return it to them (sic), for which they (sic) would be eternally grateful."
OTOH, p.117 of the English manual states:
"If a civilization has been removed from the game and you capture one of those civs cities, you have the option of annexing it, making it your puppet, or liberating it. If you do so, the civilization returns to play. The liberated civ will always vote for you in UN elections. In case of multiple liberators, the civ will vote for whoever liberated them most recently."
IMHO, this is a bug only if the phrase "eternally grateful" implies that the civilization's attitude towards you will improve. It might mean that, or it might only imply that they'll vote for you in the UN elections. In either case, the developers' intent should be clarified.