I'm not sure how else to report a bug. This has been happening for a month or few. I have the latest 2.2.2 app on iOS 11.2.2. All SwiftKey settings enabled. Repros on phone and iPad.
1. Swipe: "One two three"
> Result: "One two three "
2. Select "two"
3. Swipe: "four"
> Result depends on what app you typed in. Either "Onefour three" or "One four three "
Perhaps related, this type of in-place replacement can lead to the keyboard suggesting a word that will replace the word selected for replacement and the previous word it crashed into (first result above). It may remain confused and suggest words related to the replacement text you started entering even after tapping to place the insertion pointer elsewhere. Continues until closing the keyboard.