Pagination not working correctly

There is this query which when searched ->
https://www.phpbb.com/community/search.php?&sr=topics&search_id=unreadposts&keywords=portal

shows -> Search found 12826 matches
When going to pages via pagination till page 15, everything works fine.
But alter that when we try to visit page 16,or more the website shows this error ->
[phpBB Debug] PHP Warning: in file [ROOT]/includes/sphinxapi.php on line 706: assert(): Assertion failed
on the top of page.
Also we can't go to further pages, we can only go to pages till 15.

Earlier the issue was - https://tracker.phpbb.com/browse/WEBSITE-1319