Hey, thanks for your comment, and sorry it took me so long to reply. The rules state that when a piece in range of a friendly Ghast does move, it must flee. I read this as applying to the Go Away's scream, but readingly more closely I now believe your interpretation is more correct, as Betza wrote specifically that the scream is something the Go Away does "instead of moving." Not a bug but a misreading of the rules on my part. Thanks for catching that.
I think there are other actual bugs in the move generator. Unfortunately I do not have time to dedicate to this project in the near future; when I revisit it I would like to rewrite the engine completely in Rust.