I've been using the regular SwiftKey for years- I paid for it when I first bought it, in fact. I refuse to use any other keyboard, as it is my absolute favorite.
I started having this issue yesterday, and even switched to the beta version just in case that might fix it, to no avail. My issue is that whenever SwiftKey would normally replace some errors, typically when there is a letter rather than an intended space, the correct assumption pops up in the middle of the bar, but it isn't replaced when I hit space again, like it usually is. Sometimes normal errors aren't replaced at all, sometimes apostrophes aren't added. But sometimes it works perfectly.
Edit: Perhaps the most annoying issue- when I type part of a word and attempt to tap the bar to finish it, it only reinserts my letters. For example:
Intended word something : sometsomet
Insertion of characters like colons and hyphens is also malfunctioning.
I'm on an LG V20, and my last SwiftKey update was three days ago- about 48 hours prior to the issue starting.