libreoffice --convert-to csv using an xslx file as import converts the first sheet in the spreadsheet to csv; there needs to be a [documented] way to convert either all sheets or a given sheet. Maybe there's a parameter to the csv export filter to do this?
Moving it to NEW and changing it to enhancement. As workaround, you can use the solutions suggested here: https://stackoverflow.com/questions/35988515/libreoffice-convert-all-specific-sheets-from-excel-to-csv