This is likely not a simple matter of missing files.
I suspect a bug in the vanilla game, so this will likely require substantial modding experience to fix. If you want to work on that, I'd suggest to start by converting the bricks of the elusive targets to the entity.json format used by the framework. Then you have a chance to understand how the mission is supposed to work. Look at how the guards are spawning and check if that looks correct. In the best case, the guards spawn with the wrong outfits and you can just change the values. In the worst case, the guards are really incomplete and you need to re-implement the missing parts.
To pack the fix up, you can add the updated entity.json file into a raw folder for the correct chunk and pin its hash to the hash of the original resource with a framework.json file. That will cause the framework to overwrite everything as needed when the content pack is installed.