The problem is the posts are generated too fast (multiple posts per second), since the container speed up (we had this issue previously aswell).

This causes reply 9 and 10 randomly to be sorted incorrectly (but if it happens once, it will always happen).
I suggest instead of checking for post 9 and 11, we check 4 and 16.
This way enough time has passed by, so the posts are sorted onto the right page.