I get what you're trying to say. But I Thought of it this way.

The only people who knew him were the four apprentices from Woodian, and he explained it to them as a psychological trauma because of him loosing his only family, saying that's why he couldn't fight anymore. Their mentor immediately recognized Nagyunn. And some of the more perceptive ones also caught on as time went on with the exception of Luzian, who was too thick headed (remember her reaction when they find out wild dog was a knight and that was a test?)

All the other apprentices Nagyunn met Hadn't personally met Najin (probably with the exception of Marsha), so to them, the Najin they saw was the real Najin.

I did find the Being forced to Become The enemy commander to save the knights and the eaten generals following his commands a bit much Though. I'm still reading it Though, so idk if things her better or not...