Happened twice in RC9, no convenient save spot to refer to. No reproduction steps discovered.
Sometimes the intel tells me that an NPC colony in the core world got decivilized. No prior warning, other than maybe mission postings I wasn't checking regularly. No player contribution or even interaction(haven't visited the system for a few cycles). Possibly after pirate raids, but purely speculative.
And there was this one run where I was actively waging war against Persean League, and one of their colony was held in 0 stability for months. the intel periodically notified me that the situation at the colony in question was getting worse, probably far longer than intended(possibly half a cycle or one full cycle), before finally succumbing and being decivilized.
Based on these events, I suspect the destabilization timer and warnings aren't working as intended. Most of the time a colony(at least NPC ones) barely sends out warning via intel before getting decivilized. Sometimes they do, for extended period of time.