This is both a limitation of our software and that of Microsoft Excel.
Excel has a limitation of 65535 lines, so even with our workaround, Excel cannot be used for more than 65535 lines of data. We suggest you download and use Calc, which is part of the free-of-charge Libra office suite. Libra office can be downloaded from here: https://www.libreoffice.org/download/download-libreoffice/
However, the general export functionality is also limited in TempIT.
The workaround is to use the Custom Export function, select Text / Data Only and set the Export Destination to file.