What steps did you take to export?
Are you exporting from a view? from a dashboard?
Which option have you picked?
Where did the data come from originally (data source connection)?
Are you exporting on the server or on desktop?
I am exporting from Tableau Server from a Dashboard.
I found out it works properly if I use "English" locale on my server account. However, when I changed to "French" locale and export, the data merged into 1 column.