Oh, it looks like my fault.
The paper isn’t flat, so the pen prints on the high part and misses the low part. (But the servo looks like it’s “up”.)
The difference between “high” and “low” is very small, but just enough that the pen clears the paper in the low part.
Because the plot is lots of small marks close together, the pen stays in the sme area for a long time, so I didn’t realize what was happening.
I think that’s the answer.
I’ll try the plot again with properly thick and smooth paper and expect it will be fine.
It’s easier to see that the pen doesn’t touch the paper than to see that the paper is wavy.