On another computer, I tested with 3.4.0. Behavior is a lot better. If extrude is done at each keyframe, there seem to be no artifacts at all. If I interpolate a sequence between keyframes, there is a little bit of garbage/noise, but that is not surprising. And it is much better than 2.81a in any case. So I guess that is what I need to live with.
A workaround in 2.81a (additive drawing) seems to be to add new strokes instead of extruding existing ones. This can get the job done, just not as precise or convenient.
[UPDATE: THIS ANSWER OF MINE IS NOT REALLY CORRECT, PLEASE READ ON]
