I am not clear whether the format painter fixed the problem. If it did, are you still trying to find out what happened?
I will take your word for it that you have confirmed non-breaking spaces, but I don't see how the format painter would have changed that. If you paste the same text into another cell do you get the same result? How about if you paste text from another cell where it breaks properly into the cell where it does not? Can you delete an existing space and replace it by tapping the space bar?