Not only does the AI go twice at times I have observed the following things.
If you go too quickly at any point in the match, not just first turn, weird things happen.
I have cast ubastet as soon as I could after AI finished and the spell emptied into nothing (no barier on opponent).
Enemy Mercy will give an extra turn when it can’t. I have reviewed this and wondered if somehow a bounce from another match let yellow fall into place but it seems bugged sometimes - when you have gone too quickly the turn before.
I have matched 4 skulls in a line waiting for me just now at the end of a game with 1 dawnbringer left on AI after killing previous troop and it gave the turn back after giving damage. It has probably happened in excess of 200 times whilst playing recently.
I have fired Divine Protector and ‘extra turn’ pops up and AI steals a go before I can go again. That must have happened 20 plus times. Now I stop, wait til all animations top and then fire. It has worked. On some occasions the AI will move in two places when it isn’t even it’s turn.
I have seen AI troops take mana without their colours on the board, when I have gone before animations finish.
I believe it is to do with the animation strobes the AI has after casting spells, if you go before they finish (ever since 3.4) you can lose your turn even if you can go.