It is either a bug (though I don’t believe Kabam has confirmed that and it’s been a thing since day 1), or more likely it is just poorly worded and it’s actually saying he can’t heal back into previous phases. So when you get him into phase 2 he can still heal back to a level of health he has in phase 1, but he can’t re-enter phase 1 and make you repeat it.
Like I was just in phase 4, go kod but he went from 10 percent to 11 percent, and I'm back into phase 3 for 1 percent