Feathering

Feathering is ink wicking sideways along loose paper fibers, turning a crisp line into a tiny root system. Cheap copy paper feathers because its fibers are short and unsized; the ink follows every capillary it finds.

Fixes, cheapest first: a finer or drier nib, a better-behaved ink, then better paper. You do not have a bad pen — you have a wet medium on absorbent stock, and both are adjustable.