Model swapping in video games

The title of this thread is a new sport that can be practiced to various degrees. I've learned how to and it's actually a lot of fun. However, sometimes some games will surprise you. I'd like to evaluate some of them.

For example, In Resident Evil 5, it's possible to extract the files that instruct the game to load a particular kind of enemy. This allows you to swap enemies from one stage to another, but in some cases, it can produces glitches or faults that can't be corrected.

I made Uroborus Mkono appear in various stages before its actual encounter. The model was properly loaded and attacked the player characters as normal, but when it performs its escape move, it doesn't reappear.

For those of you who know and for those of you who don't know, Uroborus Mkono can teleport by becoming liquid and reforming a short distance in behind, or away from the player. This function works only halfway when Mkono is loaded into a different level, in that he vanishes, but reforms in a part of the level in which it cannot interact with the player characters.

I don't know if it gets stuck in a wall, or in one of the nearby buildings, but the creature reforms somewhere that you can't access either, yet, you still hear the creatures sound effects.

What prevents a swapped model from reappearing correctly in a level if it has teleporting capabilities?

Maybe its "Teleporting Capabilities" only have certain coordinates possible, and those don't match up to available coordinates in some of the alternate levels.

Why did the programmers design the creature this way and allow for people to instruct the game to load it elsewhere, then?

Loopholes in the code most likely.

You are most likely right. While we are discussing the topic, something else weird happens to another enemy that is instructed to appear somewhere other than where the location it is meant to appear in. It goes in a straight line and continues moving against a wall without following or attacking the player characters. You can damage it and the cut-scene that plays showing the creature dying plays exactly as it would in the location it is encountered.

What prevents this particular model from behaving as normal?