Hello I installed a .package mod that is supposed to override the default skin from EA as well as a no mosaic mod. For half my sims these mods work fine, for the other half, the skin seem to work in CaS and while they are walking around, but for some reason these sims default to the EA skins when in the shower. If my understanding of the .package file is correct, aren't these EA skins supposed to be removed when the package is installed? I'm stumped as to why it wouldn't treat all my pre-existing sims the same. I've deleted the 4 cache files that could possibly be the culprit, but my problem still persists. I'm stuck as to what to do next.

Have you tried reinstalling the skin package? (From a fresh download.) That sounds to me as though the nude state isn't working properly. It might have gotten corrupted in the download. (Not likely, as it does work for half of the sims, but if that is the case it's an easy fix.) Have you also tried putting it in Overrides, in case you had a default replacement skin that you thought was an EA skin?

If reinstalling it doesn't help, what did the sims who work properly have in common? And what do the sims who don't work have in common?

Skins are like everything else. If the game can't find the CC that it is looking for, it replaces it with a fallback.

(And welcome to MTS!)