I felt this deserved it owns reply..
I'm not really sure where it is coming from but here's what I have found so far..
(not tested on a clean install, so could be theme and/or mod related too)
It seems when someone is @mentioned and a topic is locked it causes the replies to appear hidden when viewing the topic as a member..
On my test, it also added a huge (padding?) space in the OP. when viewing the topic as a regular member..

.. I've also just noticed, the other replies either appear with the same padding issue (or normal, as they should - depending on the screen size - ((try resizing your browser window if you're testing))
(semi related sidenote: I also had a similar issue of missing content whilst using Tapatalk with my site and am now wondering if this other cause, was actually the fault there too..! Hmm)