Played some more again, lovely game :D
Sometimes the game doesn't make sense. bug/butterfly rule has to be "even" or "higher" based on 4 on 1 being ok, thus 8 on 4 must be ok but isn't.
There was no rule change. I did however click outside the game to write something down and then back in (browser version). This is the 3rd time in ~8 games. Not sure always related to clicking out of the game.
It just happened again. 1 on 2 ok means odd or -1 or less thus 6 on 1 shouldn't be ok. I put 5 on top of that and it was ok.
I guess sometimes npcs don't get flagged for being wrong under specific circumstances.