Disclaimer: After a recent update I was struggling with the missing autospace after flowed words. Given that this feature is now back, I have enabled it. So the comment below refers to this setup. If autospace is not enabled, then the issue doesn't present itself in quite the same way.
This problem was first described by Greg Aloe, post at 13.33 on 9 May 2017, on the forum thread about "Improved Flow (swipe) feature". Please comment if you have the same problem.
So here's the issue:
When I flow, often I'm trying to write one of three words that SwiftKey recognises. For example , say I want to write "flow". But "flow" is extremely similar to "follow". Swiftkey now just inserts whatever it recognizes, and the recognized word flashes or a split second up in the bar, but then disappears. The problem with that is that loads of times the recognized word in such problematic pairs or triplets of similar words is in fact the wrong one. This means that one then has to hit the back button (once one has realized that the wrong word was chosen) and correct. And likely one can't correct using flow, because then the same problem would just re-occur. So one has to tediously type out the word very carefully.
In previous versions of SwiftKey, a selection of words would appear in the bar, say "flow" AND "follow", and another word. Then I was able to select the one I wanted, if the wrong one had been auto-recognized.
This problem happens to me ALL THE TIME. It means that I'm super slow when using flow because I keep having to correct things (and switch from flow to typing). I think soon I will stop flowing altogether because it is just such a pain. But then obviously I can use a different keyboard -- which is sad, since I used to really love SwiftKey.
The issue seems to be that SwiftKey developers have become too confident in their algorithm. There simply are some words that are too similar to distinguish when flowing. And in these instances, SwiftKey should present a choice in the bar, like it used to, rather than selecting one that is probably wrong.