I noticed this happens with new products only. For the old products, I had regenerated the SKUs with 13 digits and they are all 13 digits like 5869718383469-01, 5869718383469-02, 5869718383469-03 etc. However, whenever I add a new product, the SKU that is auto-generated has these dashes. I have also set “-” as the separator for variant SKUs.