Whoa, that's a lot of questions ^^ !
First I want you to be assured that
this is not a major issue to me. I'm perfectly aware of the fragility of CK2 saves when it comes to updates, so if my game has been rendered unplayable (which is not the case as of yet), I can totally survive it and start again

!
Let's answer your questions one by one and in an appropriate order :
- "
Was this a loaded save, or brand new with the latest mod version? And do you have the correct checksum?"
My game version is 2.3.4, and the checksum is BKIG, so everything is correct on that matter. However, my Zoroastrian empire game was started a week ago, so this is an "old save" and there are chances that this would be the reason of this whole mess. More details afterwards.
- "
Did your ruler die?"
No. Last time I saved this game was thursday evening, before the last version release. My former emperor JUST died and a new one had inherited. There was no "imperial decadence" icon at this time, it only showed up when I loaded my save today. This comforted me in the idea that things were going smoothly with an old save.
- "
There is no "reduce imperial decadence" ambition. What are you referring to?"
Sorry, I mistyped here, I obviously should have written
decision, not
ambition. It is shown on the screenshot.
- "
How did it skyrocket? And what do you mean "next time I gave it a check"?"
First time I saw the icon was upon loading. I had just read the patch note and I wanted to make sure it applied correctly to my game, which it appeared to have. At the time I started playing -circa 1263 in game- my decadence value indicated 10-20, with some minor maluses like -8,0% levies and -0.4 taxes or something like that (I'm just giving what numbers my memory can somehow gather, this is not essential anyway). 3 years later, in 1266 when I decided to give a try to the new decision to reduce decadence, I saw the number 11999.
The ONLY important event that happened during these 3 years was a vassal dying and the inheritance of a vice-royalty. nothing else.
- "
Every time the Imperial Decadence value changes, you would be notified via the message box. Unless you're ignoring the message box completely, I'm not sure how it could leap from 10-20 to a value in the thousands. That's rather odd, to say the least."
This is odd indeed, but since the save was an old one, there might be no reason to look any further. I never saw any kind of notification regarding my decadence value in 3 years. And I DO check my message box all the time of course.
Anyway, once again,
this is not a major issue to me. Just a silly annoyance I can perfectly live with seeing how all-powerful I have become anyway.
The game was becoming a bit boring as I had absolutely no competition at this state and it was essentially managing suceptibilities and keeping my throne safe decades after decades.
I'll continue as much as I can with it, and if it becomes seriously unplayable, meh, I'll just start a new one, no big deal really.
I just reported this oddity so that you would know that it was out there for old saves

.