It is being assumed that they have no Dor on Roshar...
What if the power loss is like a parabolic(? not sure if that is the right word) effect. Imagine if, they travel so far away from Elantris that there power grows again like the opposite pole of a magnet (kinda like how its cold in the north pole, warm in the equator, then cold in the south pole.)
OR
Sel is somehow "close" to Roshar, and every place in Roshar has a corresponding place in Sel.