When the grid contains one sheet, more than 65535 records could be added and then exported to xlsx without a problem.
The data corruption happens when the second sheet is used.
Apparently, to reproduce the issue adding second sheet is enough. No need to add any data to it.
In the attachment the new simpler version of the test project. Test.zip (2.7 KB)
Click Start button, when the "Add Second Sheet" is off. Notice how fast the grid populates and all data is correct.
Enable the checkbox. Click Start again and observe that generation takes much longer. Data in the first rows is wrong. Switching between sheets takes a long time.
Have not seen any reply from support yet.
Let me know if I post in a wrong area or you need any additional information.