As has been pointed out by myself and others there is at the present moment a discrepancy between how the Sorcerer and Self-Taught mystic are described lore-wise, and how they work mechanically. Lore-wise these two Approaches should be more susceptible to Corruption, since they lack the training from the magic traditions. However, right now they are BETTER at avoiding Corruption than the traditions. I guess the reason for this is due to the conversion between the different game engines. If I recall correctly you would get a point of permanent Corruption for every new spell/power you got, unless you first had spent xp on joining one of the magic traditions. So in the old rule set you would gain power more quickly, but at the cost of permanent Corruption, meaning at higher level games Sorcerer and Self-Taught mystics would really start to be affected by their permanent Corruption. In the 5e rules you no longer get permanent Corruption while learning new spells, so this might just simply be an oversight in the rule conversion (and if it's not please correct me).
A suggestion I have to align the lore and game mechanics in RoS, without completely changing the current traits for the Approaches, is to simply add a new trait; something similar to this:
The Price of Power: By not following the careful studies of the magic traditions you have quickly gained power and learned how to cope with the effects of temporary Corruption. However this has left you vulnerable to the long-lasting effects of Corruption. You start with one point of permanent Corruption at level 1, and you gain an additional point of permanent Corruption every time you gain access to a new spell level.
Of course these numbers can be changed accordingly to make them more balanced.
What do you think?