Very likely, every time you type "soon" and it corrects to "son", the probability of the change gets increased. So, the more it errs, the more wrong it will be in the future.

About it only replacing after you start writing, the probability of "son" must grow faster than the one of "soon" after you type "so". If there were such a huge bug, you would be seeing a new word every time, not always the same one.