Her affection to Kurt on the other hand is not likely a mistake, if one take time to think about it (that and it has been explained by several in this forum).
While Arika was affected by the plant monster (EV3), and so was Rietta and to smaller degree, Terena, but the latter two has it worse than Arika.
Rietta was already in the Guru's grasp, from both her youth and the monster that attacked her early in the game.
And then she and Terena were caugth in a trap (EV23) and were remodeled and injected by the goblins with brainwashing fluid (Rietta) and lewd worm eggs (Terena).
Rietta's brainwashing is pretty oblivously why, but the worms inside Terena is absorbing her magic and knowledge, and turning it to pleasure, making her turn into a dumb whore.
in comparison; Arika only has her increased sensitivity to her body(Plant Monster) and her mostly her clit, with neither has anything to do with changing her mental state, at least not in the same way as in Rietta and Terena's case.
She was likely planned to be modified by the goblins too, but Kurt saved her from that fate (not it really matter in the long run in this game)
In short, Arika's corruption is slower do to these factors.