my well filled rival list is empty, though the last attacks are just 30 minutes ago. I had dunno maybe 10 squads in that list and suddenly they are all gone...
if the rival hasn't attacked u for 24 hours, they are off that list, but i'm not sure about the one that was 30 minutes ago
i was on the hitlist and someone attacked me 27 times and now he's not on my rival list no more, was around 1 hour ago
I believe when they do updates it resets the rival lists as well. Mine was also reset & I lost my rivals.