I noticed this issue today while reading the forums. On a multi-page thread, using the page button along the top and bottom, or using the 'next' link to flip the page, causes some posts not to be displayed. I actually have to go to the address bar and modify the "&start = " portion of the web address to bring up the missing posts.
The problem is occurring with both the web interface and with Tapatalk. It's also intermittent: the problem shows up on some threads but not others.