I was not able to reproduce the issue I was having, however it was consistent enough that I had to remove this mod.

Basically, seemingly randomly, I would lose the bonus the mod was providing to my inventory spaces. I see an old thread about losing on death, but I have not died in my save yet, so it isn't related to death.

This did start happening after I started using teleport tech in my Nullius playthrough so maybe it had something to do with the teleport tech.

Sorry I can't provide info on how to reproduce the issue, but removing this rpg system mod immediately fixed my inventory.

I've had a similar issue when using a jetpack from the jetpack mod, did you use one too?

Mine resets too after using jetpack from jetpack mod

I have just tested here mods SE + RPG alone: nor dying or using jetpacks caused the inventory loss. Must be something else... another mod ???

Losing the inventory bonus when changing armor too, but I do have a lot of mods so don't know which one does that.

I have the same problem, I already reinstalled it and it didn't work.
I tried using the /rpg-reset-all /rpg-reset-points commands, it didn't work either.
continued bugged inventory, even paying points etc, it always returns to the armor default.

mod that I used and updated, before inventory bug,
https://mods.factorio.com/mod/helmod
https://mods.factorio.com/mod/compaktcircuit
https://mods.factorio.com/mod/RitnLib
check later if it is not in conflict.