I have not found anything in forum: however, was working on this and here is what I think is happening.
When the focus keyword is entered in the dialogue box, the tool starts searching and scoring the page or post url, title, headings, subheadings, content, and any other media tags from the page or post.
When you are not hitting your target keyword, the SEO tool will spin its wheel next to focus keyword dialogue box and not stop.
When you achieve some level of SEO rating for the page or post, you will get a yellow-greeen or green SEO indicator on the page or post, and the little wheel stops spinning. At least it stops after I save the changes.
Based on this experience, I believe the tool is working, but when it does not have enough data to produce the SEO score or rating, the wheel spins till you make improvements on the score.
Let me know if that is similar to what you have happening.