I think the easiest way to make sense of this is to have a look at Owen's report on the subject, back in Shadowbringers.

Quote Originally Posted by Eikonic Corruption: An Overview
This report seeks to provide an overview of the mechanism by which eikons corrupt and bind men to their will. The phenomenon we call “corruption” refers to the alteration of the aether of the soul. Said aether ordinarily exists in equilibrium; no one element being more prominent than another.

But when a subject is exposed to the aether of an eikon, this changes, their soul taking on the properties of the entity in question. By way of an example, exposure to the aether of the Fiend, Sephirot, would cause a subject's aether to become aligned with the element of earth. As a consequence, the subject would attain heightened affinity with earth-aspected magicks, as well as preternatural levels of endurance. Repeated exposure would further enhance these traits, ultimately altering the subject's very flesh.

The changes undergone by the subject are not solely attributable to elemental alignment, however. Further testing has determined that the subject's soul becomes umbrally charged, or stagnant during the process. This stagnation of the soul has the effect of diluting the sense of self, rendering the subject vulnerable to the will of the eikon, the thoughts of whom come to consume their entire existence. In this manner do the corrupted become worshipers of the eikon, their prayers serving to further empower the entity.

Having discerned the mechanism of eikonic corruption, my colleagues and I set out to develop a material capable of shielding one from its effects. An endeavor in which we were successful, albeit it at great cost... In the course of testing, many of my assistants ─ good men and women all ─ fell victim to corruption. And in accordance with protocol, they were summarily put to death.

I subsequently submitted a proposal to investigate potential cures for corruption, but it was rejected ─ deemed “nonessential” by Lord Amon. Though I knew it would be fruitless, I protested the decision. And for my impertinence, I have been stripped of rank and title, and will shortly be expelled from this facility. In all likelihood, this report will be expunged from the archives. Nonetheless, I record it in the hope that one day, someone will undertake to do that which I could not, and find a cure for the victims of eikonic corruption. End recording.
Fandaniel/Amon even goes so far as to bring up the report himself in Endwalker, to help jog your memory about it.

'I come all this way to admire one of my splendid towers, and what do I find? Fools attempting to ward off its tempering influence with magic trinkets. I seem to recall a similar experiment in ages past. What was that man's name...? “O” something... Oween?'

I think that Ryu's explanation from earlier in the thread is the best one. Tempering itself is not an all-or-nothing phenomenon. It's a bit like radiation exposure, with dose dependent effects ranging from enthrallment to transformation. You can have protective factors, like the Echo, Resonant, or the Blessing of Light at your disposal to help reduce the absorbed dose. I wonder of soul density plays a part as well, because there's more soul and more memories that you need to corrupt first to override the individual's sense of self.

Likewise, Venat also mentions that her 'traveller's charm' only partially protects you from the effect:

'...Primals, you say? I'm not familiar with such beings, but if they 'enthrall' by warping the balance of the aether, then yes - the magick would offer you a measure of protection.'

I'm sure that we'll see more discussion on the subject leading in to Meracydia and the Void.