Thanks for playing and sharing your thoughts!
Just to clarify: the enemy deletion isn't a bug—it's actually my fix for the performance issues! Without it, the frame rate was tanking down to nearly 0 FPS (though on Level 5 it drops to around a minimum of 20 FPS).
I had the despawn distance set way too far before, so right now enemies despawn at 3,000 pixels to keep the game playable. I might bump that up to 5,000 pixels later after some more optimization. Thanks for the feedback!