Not only in GAC, I had this problem in Conquest too.
I don't have Starkiller myself, so I only see this with SK in defense. It does not happen everytime though, and I don't know why. But still very frequent, around 90% of the time I'd say.
So, in conquest I avoid any node with Starkiller, but sometimes it's not possible, when the path forces you to fight him. I lost an entire day once just to pass him, not because I couldn't beat his team, but because the battle freezes!
And it happened again just 5min ago in GAC, very very frustrating. I think I'll lost this round because of that.
And I didn't even get the 10 points for first attack, even though I did 1 battle (see screenshot).
I play on my Android phone (Vivo Y70).