The answer is simple: Grock got feeblminded in the previous battle. I don't remember how he got feebleminded in the battle with my Satyr Warriors, but I distinctly recall checking him before the battle was over and seeing he was feebleminded.
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I also checked him, during and after the battle. He was not feebleminded. We must have watched different battle replays (maybe yet another example of my Mac biting me in the butt).
However, if he were feebleminded last turn, the magic troops under his leadership should have left his command the turn after the battle, not after moving with him to a new province 2 turns later.
Well, now is your chance. The feeblemind should wear off in a couple of turns, and slowly ever so slowly, our research is creeping up on some significant milestones.
Now there are two feeblemided pretenders, and a lot fewer white centaurs.
On the down side, Grock is so horror marked that Kurgi and his ilk are likely to be making monthly visits. Maybe Grock will take them all, but I kind of doubt it.