Okay so 'Well Built' disappearing is easy to explain, that bonus applies only to Non-Elite units. When the Hunter evolves into a Siren it becomes a Elite unit, and hence losses the +2 Armour. The disappearance is a lot more complicated, it currently disappears when the unit changes Tier. So Chrysalis to Spawn and Hunter to Siren are evolutions going from Tier 1 to Tier 2 and Tier 2 to Tier 3, which causes an issue. I'm going to have somebody look into that one as it will require code support to fix.