CSV is a textual data file that stores data in columns, but no formatting at all. That includes the cells borders. So, when you import a CSV, it imports into Calc with default formatting, which is no cell borders. These invisible cell borders are displayed on screen with a light grey color by default (to make user’s life easier) - but they are still invisible when printing.
You may simply apply cell borders by selecting required range, and using Borders
button on toolbar. But remember, that the formatting would be lost if you save to CSV, because that format is unable to store formatting. So if required, save the file as ODS.