That's a fair point I didn't fully consider. The impact levels were mostly chosen with considerations for their actual birth in mind, where their whole impact during all of history comes into play. But it also influences collapse priorities (rarely: when vacating the last available slot) and respawn priorities. This might happen at a time where the historical importance was not as high as their original historical importance. Greece is a good example of this.
My first thought would be to assign a lower impact to civs that are already past their fall date, but that's not as easy either: China retains its impact for all of its history, others lose it somewhat, even others lose it entirely. I already recognised that the concept of a "fall date" may be too crude anyway, considering that it forces e.g. China to be considered "past its fall date" for all of its history. Maybe the correct solution to this problem is to have "fall/decline windows" that can begin and end, just like there are for respawns.
This is further enhanced by a fifth impact level, named "Player". No civilization inherently has this impact level. Instead it is always the impact level of the civilization controlled by the player, as well as other civilizations considered "important" for that particular civilization's gameplay. Those usually include neighbouring civilizations as well as conquerors etc. that should be expected to be relevant for the game experience of that civilization. For example, America includes its neighbours Canada and Mexico, but also the colonising civilizations of England, France, and Netherlands. Since "Player" is considered higher than every other impact level, this virtually guarantees that these civilizations will always be in the game when you pick a certain civ to play.
Best I can do is one or two turns after the war declaration. I guess that's not helpful. Also after reading recent posts, I suspect it's not compatible with the latest test version. In future if I have an issue I'll be sure to keep track of the autosaves.Please upload a save from before the war declaration so I can investigate.