When you type a particular word, and you want it to retain that word exactly, are you pressing the "checkmark" key?
If you don't the predictive text can do some weird things at times.
I personally have no issues with the way it works, the description of how it works states, "choose a word that is shown, or use the Check Mark to retain the way you typed it.
(if you hit the space bar instead of the Check Mark, it does not always retain the word like you want it.)
Using that practice, I almost never have any problems. At times, the exact word I want has like 3 different suffixes appended, I just choose one of them, back to erase the offending letters and then the predictive text will present the next set of predictive text words, and 99.9% of the time, I get the one I want.
It seems that the base word + "ed" is not deemed to be a first level choice, it is a 2nd level choice. Thus me backing out and erasing "ly" or "ing" and then adding "ed".
no matter how it goes, it is so much faster than me typing out the whole word character by character.
I have given up on swipe, my fingers just can't seem to learn how to use it.