This arises because Excel automatically converts all cells to a datatype it deems appropriate. In this case, when opening the CSV journal report, a number. Effectively removing the leading zeroes.

To stop this from happening, instead of double-clicking on the CSV file directly, you can import it into Excel as a text file. This will tell Excel to treat all fields as text.

To achieve this, follow the below screenshots:

Here's also a great video of how to ensure leading zeros are still kept in Excel: