This calc used to work with Linux Mint 19.3, but since upgrading, the cells imported from a .csv file give a result 0 (zero) when trying to sum a column.
This might mean that either the new OS/LibreOffice has a different locale (and thus, different number recognition rules are active), or your csv is different, or simply your old setup had Text Import dialog set up once correctly (to use proper locale), and that was saved and reused.
Problem solved, it was the locale settings between the original calc and the one on my computer - two different English language locales.