I believe it to be arena power, too. The reason people have their easy 12th node disappear is because top arena teams power keep increasing thus having an affect on your 12th node calculation. I got easy nodes 12 and 11, and one day my node 11 got hard out of no where. Well I had a gw refresh the same day my node 11…
I always get to node 11, then GW says oh no sonny you aint getting to node 12. I don't mind stage 6,9,10 being hard but every time I get to stage 11 there is RG laughing at me behind 50k of protection like angry joes evil corp character.