I have multiple requests into Amazon to push out updates to everyone, and to help me get various issues like the Kindle Cloud Reader fixed.
However, authors don't have any mechanism to contact them directly. We email and then wait for up to 24 hours.
As it stands, until I get them to push out to earlier purchasers, only new readers get the newer versions.
People who do not see the Updates Available button need to do "contact us" Give them the ASIN of the book B01N6DL30K
Tell them the author has published versions that fix readability, they will push it to you and your button should work.
Again sorry for the inconvenience.