In the Quality review, the number of grammar issues never goes down, even after I polish the chapter. Why?

Update to version 1.9.15. In earlier versions the proofread was asked for a list of issues and mixed real errors with matters of taste, read only the beginning of long chapters, and "Polish chapter" rewrote the text freely, which gave the next check new sentences to comment on; the list never got shorter. From 1.9.15 the proofread separates real errors (grammar, spelling, punctuation) from optional style suggestions, and only errors are counted. It reads the whole chapter, gives the same answer when you run it twice, and reports a clean chapter as clean. Every finding is checked against your text, so it can only point at words that are really there.