As the title says, the Corporate Embassy corporate building requires the planet to have a Planetary Capital (or better), but cannot be built on planets with an Enigmatic Palace (I've yet to try it on planets with higher-level ACoT capitals, but I assume it'll be the same for them). Every other building I can see, corporate or normal, with a capital level requirement counts the ACoT capitals just fine. I assume this is simply an oversight that got missed.