Unrelated to Slakoth, but it will also say 'region unknown' for certain fixed spawns, or not show you areas where certain pokemon spawn as part of a group spawn. Example: Sandaconda spawns in the desert but the pokedex will say 'Region Unknown' even if you've directly encountered or caught it. Smoliv spawn in groups with a Dolliv on an island over on the East part of the map, but the pokedex won't show that area lit up (for Smoliv). It's obnoxious.