OK here's the issue!
It looks as if the 5.16.7 update has changed text selection behaviour by design. You cannot drag a text selection across screen breaks for any ebook that is not an 'official' Kindle book. So, an epub converted to AZW3 using e.g. using Calibre will not work correctly.
However, using the Kindle-to-epub converter seems to restore the correct text selection behaviour.
This is a bit of pain and I regard this is a retrograde step and although I understand it from the viewpoint of the Amazon 'business model' I don't like it one bit!.
See also my new post to the Calibre forum:
https://www.mobileread.com/forums/sh...d.php?t=359511