I have had multiple instances of invisible (or rather not able to see the model due to it being underground or off screen) enemies. The clip below shows an example of the root of the problem
Take note of the animation the Winged Fire enemies play when they come back from Anomaly land. They replay their spawn animation when returning. I couldn’t capture it in action in a monolith but this animation sometimes will fail to animate causing the osprix enemies who fly in from off screen to be invisible and able to attack me with no animations shown. I’ve been able to fix enemies by stunning them sometimes or letting them start a new attack animation.