So far, Amazon has shown all the KU books I've completed as having been read.
There was some discussions years back (Quora and a couple of other sites) about the algorithm used for calculating valid pages read and the conclusion in one discussion that after some automated page flipping tests, you needed to spend an average of 5-6 seconds on a page for them to count as read (the number seemed to vary a bit with font size and margin so they may use a number of words per page calculation). If you read at 4000 WPM, this might be an issue.