First, just because it isn't explained doesn't make it a plot hole; obviously Avellone and the other writers did consider this matter, since they wrote material hinting at an explanation but unfortunately got cut from the final game. So at the very least there is an explanation, it just didn't make it in. That's not a plot hole, that's just unfortunate.
Second, you seem to be ignoring or forgetting certain bits of dialogue that are still in the game that still hints at possible explanations - and indeed, there's always "Kreia was lying", but I'll admit that's not much better than "a wizard did it". But anyway, here's a bit of dialogue:
Originally Posted by Zez-Kai Ell
A bond between two living beings is not something easily broken. It is not a choice... it is like breaking a feeling. Like turning away from the Force. To break a bond, your feelings would have to change, or one of you would have to die - but even then, the bond wouldn't go away, it would simply... it would simply be empty, a wound.
1. He says breaking a bond is not a choice, like turning away from the Force, and yet the Exile did this already.
2. It's not the act of falling to the dark side that would break the bond, it's the feelings that form the bond changing, causing it to break; Kreia betrays the Exile, the Exile's feelings about her change, simple. What complicates it, as I mentioned earlier, is that the Exile needs
these bonds to survive. Kreia talks a lot about how she wants to make the Exile overcome Malachor, become stronger than her, and so on - and killing her is the final test.