This has now happened to me a few times while using the line tool and today I managed to reproduce it.
This is on a grouped ink layer, the group is set to multiply mode.
It isn’t something that happens when I work on my initial line art, but rather something that happens when I make adjustments while/after coloring or rendering.
The artifact is embedded in the save file if work is saved after it occurs.
The brush used is the ink gpen 25 from the krita 3 default brushes.