This was an issue in previous versions and was also reported. Obviously no one fixed it yet.
OneBlackbird
You're not alone. I really wish they would've waited longer to release Elemental. It's very clearly unfinished and while it's playable, it is pretty much the same thing over and over. Just because a game can be played doesn't make it finished and this is what disappoints me the most. Had I known the game would be released as it is/was, I never would've dropped $50 on it and pre-ordered.
Yes, the autoresolve seems to make casters cast as many of the highest cost spells as possible. I had two sons with around 200 essence each and whenever I autoresolved, they would end up with 100 or 150 and eventually 0, which was incredibly annoying.
Just don't try to get him to give you any gold. I made this mistake with an AI player that had 0 gold. I thought "WOW, a bug that's actually going to temporarily make my game easier." Wrong. I ended up with -2218738716387683678128 gold.