Before installing this mod, store your extra items and remove your power armor. Save. Then activate the mod, load the game, and save again once you confirm that it is indeed saving the extra inventory slots.

Installing mid save deleted the bottom line that was my power armor extension on "Modular Armor" upon loading the save. I haven't spent time trying to figure if there's a conflict.

It's possible it's a compatibility issue but even if it's the only mod if you add it to an existing save wearing armor that expands your inventory and have those slots have items it either deletes or spills those items onto the floor. It's a combination of how armor is coded to work and the API limitations (or lack of knowledge). I have to set the armor bonus to 0 then manually add the slots back with character inventory bonus, which means that when you load the save with the mod added for the first time you have fewer slots than without the mod (due to changing the armor prototypes).

I've not validated compatibility with many mods this was mostly a mod for myself so compatibility testing with other mods that add armor is limited to me playing with them. I know for a fact that the mod in its current state is actually incompatible with nullius.