Any update on this? I'm having the same problem with this. It doesn't stop with sions either. I built an ogre camp and can't train ogres either. (Darklings train fine however, and have as long as I've been playing)
Sorry, I spent over an hour searching for a post on this before posting this, https://forums.stardock.com/395809, but apparently you have to use the search engine at elementalgame.com, not the elemental forum on stardock.com. Oh well, if someone could merge the two posts or just delete my duplicate thread thanks.
I too was playing custom sovereign with custom fallen faction, however my first game was as custom sovereign with Resoln and I had the same problem.