Update, the age old "restart" seemed to have solved it for now I think, it is now moving down again.

Hm, it does go down, but feels like some hidden mechanic not being shown keeps adding it every eyar, it goes down every month, no events has happened and I was at 54 corruption now at 59... Not sure again. =P

It is month of may the 20th each year it happens, been happening for almost 10 years... Is the game file bugged perhaps? Or some hidden mechanic I have failed to notice?

Same here.
My increases around june each year. The date is different all the time, but it's alwaysa june. I have a regency council and I'm seizing land from nobles. Maybe its a hiden mechanic? Or is it a game breaking bug?

It's probably caused by realm managment activities (such as siezing land from nobles) that you have active. They apply the corruption during their annual maintenance cycle.

Originally posted by RivtalDM:

It's probably caused by realm managment activities (such as siezing land from nobles) that you have active. They apply the corruption during their annual maintenance cycle.

Thanks
How long does "the siezing land from nobles +10 unrest " modifier stays active? The nobles in the province a down to the wanted percentege for quite a few years now, but the modifier is still there.

Originally posted by RivtalDM:

It's probably caused by realm managment activities (such as siezing land from nobles) that you have active. They apply the corruption during their annual maintenance cycle.

I did use it once in 1 provinse, but that was like 20ish years ago, I thought it would be like a 0,25 increase or so, anyway I assume it is as you say and a choice I made somewhere I will go back and see if I can reset somehow caue I had it play and just watched for decades and it just never stopped increasing.