This is not an issue with mendix :) (I simply copied the number you gave to a brand new excel file and it see the same thing)
Check this documentation link from MS Excel (Which is in turn true for CSV files as well)
Once downloaded you have to manually correct the number formatting.
I know it is not desirable for such numbers to be stored as STRING in mendix itself which kind of solves when you export. But it takes away the underlying need for numbers.
Good luck,