This is not a bug, this is a feature. One of nano's best in fact. What you say is not true except in the case of consecutive cuts. So this is a way of cutting

This is not a bug, this is a feature. One of nano's best in fact. What you say is not true except in the case of consecutive cuts. So this is a way of cutting multiple consecutive lines without requiring a visual selection. If you don't want that just move the cursor once before the second cut.