I’m not aware of any, “faith healer” who teaches that you will always walk in perfect health. Just like I don’t know any pastor that teaches you will always walk in perfect freedom from sin. Or a marriage counselor who teaches you won’t ever have marital problems. Or an evangelist who says you will win every soul you preach to.
While these may not all be exactly the same in comparison, the point is that we fall short in all kinds of areas. I don’t know why the area of faith and healing would be different.
Even if it were, at best this would be a classic example of a genetic fallacy. Just because Johnson falls short of the message he preaches (I’m speaking generally since I don’t have his official position on these topics in front of me) doesn’t necessarily mean that what he says is untrue. The truth is the truth, no matter the flawed vessel that speaks it.