I don't know what happened, but at the start of chapter 9, M'Khiin had 276,896 XP and at the auto-save at the end of chapter 9, she had 262,604. She somehow lost 14,000 experience.

I've kept her on my team full time since recruiting her, so I don't know what happened. She's the only character this has happened to too, as all the other characters from chapter 9 start to finish have gained almost the exact same amount of experience while Gobby over here is at a 14k deficit.