Soooo corruption resets between game sessions?

I am certain I had 144(ish) corruption on the empowered Monolith: Stolen Lance. I beat the boss (as I'm farming Wings of Argentus).

I then left the game.

Today I log back on and walk over to that timeline and it's back to 100 Corruption. Is that how it works?

I'm playing Offline if it makes a difference.

If this is how it is then I don't understand how people farm 500 corruption or whatever. I find it quite repetitive and not challenging to beat the <150 corruption monoliths at this point with my character and I was hoping to build it up, but if it resets every time then I'm not doing that.

Let me add that I love the game, just that it's too easy at this point with my build (I'm level 95 with decent gear - I've played mostly about 2 years ago and only came back the other day so I'm rusty on the game mechanics and those that changed).