Firstly, I’m told that cast I’m told that cast out does autocast if your opponent has no creatures but it asks you if your opponent does control creatures so you can refuse. Is that correct?
And a little bit of testing here and there seems to imply that I’ve been making changes to actual deck slots rather than event deck slots, so that’s probably what happened in the second case.
It’s a little weird. In my experience, cast out autocasts only when there are no creatures on the board at all. If there’s at least one (whether it’s yours or your opponent’s) it’ll prompt you for which you want to target, and remain in your hand if you say “not now.”