Ariel Walsh randomly disappears (along with Xavier Alvaro) in Isla Paradiso

poppingmythik​  Both of these are homeless sims, and the game will by default delete homeless sims for any reason, or no reason at all, usually replacing them with new ones.  If you want the old sims back, you can save them from a new copy of Isla Paradiso and transfer them, but that requires a bit of work, or else a mod (MasterController is sufficient).  Let me know if you're still interested.

Using NRaas StoryProgression will prevent the game from deleting resident sims, but not homeless ones.  The only way to protect them is to move them into houses.