The Emancipation Proclamation National Wonder works differently from Great People's "Abolish Slavery" action. Please see the capital city (Uruk). If you take a Great Person (e.g. a Prophet) and do "abolish slavery", it destroys buildings, and converts slave specialists into Immigrants and traders. If you instead build the Emancipation Proclamation (1 turn), then the slave buildings and specialists remain in the city (but the worldview -- slavery and worldview -- human sacrifice disappear). Is this intended?
Also, a couple of notes. When converting slaves to immigrants, if you have fairly large cities, the immigrants are useless (since you can't settle them in cities above 7), so you essentially have to kill them off. Converting production slaves to traders is also kind of iffy, since you could, in theory, get a ton of those single-use free traders, which would be nigh-useless, unless upgraded. Would it, perhaps, be a little more appropriate to upgrade settled slaves to citizens instead?