The maximum number of rows you can export to Excel is 65535. If you need more, use the CSV export. This was already a constraint at runtime in previous versions, but now turned into a property of the button.
To me it just stopped at row #500 and it took me awhile to figure out how to do this even that it is extremely simple... but remember that the MaxRows property must be set/defined before data is put into the spreadsheet otherwise it has no effect.
Dim i as Long
.MaxRows = 65535
For i = 1 to 15000
.row = i
.col = 1
.Text = "Text to spreadsheet"
.ExportToExcel strPath, strSheetName, "log.txt"
Please see this answer from a general point of view in VB and not from a Mendix-related point of view.
Hth Jess, c++ specialist on unsafe vb ground