Sim household members randomly dissappearing?

@Kiddope92 For the households with the disappearing Sim, are you keeping them in the My Households tab or the Other Households tab in your Manage Households menu? There is a limit on the number of Sims you can have in your game (I think it's 800, not including ghosts) and if you reach that limit then the game will start culling Sims. As far as I know, if they were culled then they are permanently gone. You can import a new version of that Sim if you have them saved in your Library, or if you have an older version of that Save file you can go back in time to before they were culled.

The only way I know of (without using mods) to prevent a Sim from being culled is to move that household into the My Households tab in the Manage Households menu. This should exempt them from the culling process. Sims in the Other Households tab are not safe even if they are living in a home, even if they are one of your old playable households. You can move the household from the Other Households tab to your My Households tab by clicking on the heart icon in the lower right corner of the household tile.

Some mods like MCCC have the option to flag a Sim to never be culled. MCCC also allows you to generate a report to show you how many total Sims you have in your game to give you a little warning about when you are getting close to 800 total Sims.

Here is a great site for more details on how the culling process works:

https://sims4.crinrict.com/eng/2016/06/culling-what-is-it-and-what-does-it-do/