I started a new game with the 1.08 patch and then installed this over it.
1936 Germany, did event chains to get me into WW2. Italy went after Greece and got DOWed by the Allies, I accepted them into the Axis which gave them a separate war with the Allies.
1941 I did Sealion and got the UK surrender event, installed Mosey to get UK as a puppet. After I did this everybody left the Axis. I retained my puppetted countries but inherited the UK's separate war against Italy. Since everyone left the Axis, including Germany and Italy, I found myself at war with Italy, which put a crimp in my plans to invade the Soviet Union.
Is this a fluke? Has anyone else noticed anything like this? What could cause everyone, including Germany, to leave the Axis after the event "Victory in the West"? Is that a criteria for the Axis, staying united until Victory in the West?
I... er... what? I have no idea why that would happen. I've never experienced it myself. It's natural that Hungary and Romania leave the Axis to settle their differences and then rejoin... but I'm unsure if that is what caused your issue. My bet is on Italy's "separate" war with the UK.
One solution is to add a peace command for the UK with italy.
The problem with this is... what if Italy is fighting a TRULY separate war against the UK? What if Italy is also attacking Germany? That solution could cause problems on its own.