At first I thought it was unable to see where each line ended, so I fiddled around with the line break formats.
Tried Unix, Mac, Windows and none of them worked, only the header was visible.
So then I tried creating a new CSV file manually and now DataGrip was able to see the content.
I found this weird so the next step was to use a long line (>100 chars). Lo and behold, this line did not show up in the preview.
I'll attach a screenshot and the DataGrip window with a text editor side-by-side.