In the ticket list search with more than 5000 results, when an export is performed only the first 5000 rows are exported, the rest are excluded.
There is an option "export_max_fetch_rows" that controls the amount of rows export. By default, the value is 5000.
Modify the option "export_max_fetch_row" value from Options Manager under Web in Administration tab after logon web interface as Administrator. You must recycle Service Desk services after modifying the option.