I bet if you were to open the CSV file in Notepad or similar the numbers would not be in scientific notation. The issue is with Excel. Excel is causing any number over 12 digits to be changed into scientific and their own function for changing it back is not accurate either. The inaccuracy is what worries me here... I only had the one number that was not correct but one number is too many in that situation.
Every solution I see posted says to just convert the numbers in the way I mentioned above or to use a formula in a secondary column to convert it into text, "=CONCATENATE(A1)" in column B. But again the accuracy worries me. After seeing one number be wrong that really shakes my confidence in using Excel at all for converting things. I did see a suggestion that said to format the cells BEFORE you paste the data in and that may work better.
That fix might work if you do it on your computer and then mail the file out, I assume the secondary column would work this way. However some of the fixes may heavily depend on the users Excel settings that opens the file. So it might be impossible to fix without having the users make the change themselves but you will have to test that to be sure.