Though the releases for this series are months apart from each other I suppose here is the reasoning behind this chapter, this series also has a somewhat strange translation in various instances where some things aren't spelled out clearly enough.
[ul]Church is supposedly unaware of the side effects of "immunoboost" thinking that it has the same properties as the Heal spell hidden by the Church. They hide it in-order to preserve the image that only the Saintess is capable of using the legendary Heal spell.
Church then sends out the "Saintess" Isabelle to travel to various region to spread out the fame of the Saintess and the Church by correlation by using "immunoboost" on various people who are injured.
In this world there is difference between "Recovery magic" and "Healing Magic", Immunoboost as the name suggest uses the person's own life force/body to temporarily fix in injury. It will eventually catch-up to them again, as their body tries to compensate from the shock of whatever injury they had. While Heal uses magic to aid/supplement the recovery process without a burden being placed on the body.
Up until Charlotte fixed Okis, nobody seem to draw a correlation to the death as being as direct result of the use of Immunoboost, i.e. Charlotte with her editing skills is able to see in-depth the conditions of other people so before she came along everyone assumed disease or unexplained causes.
Its implied that either Isabelle stubbornly relied on the use of immunoboost, not taking heed to the spirit of light who came from Charlotte to help her, Isabelle is constantly wary of Charlotte and her intentions because from her perspective being the Saintess is something to be proud of because it makes you special in the eyes of the people, alluding to the possibility she took the Saintess title from Charlotte somehow and is worried that she will somehow realize this and try to take it back. [/ul]