Windows 10 Home 1909, Designer 1.8.3.641 (and 1.8.4.650 beta).
Expanding the Stroke of a simple line gives unexpected results when performed repeatedly.
See video (Designer beta recorded) for two examples, one straight and one curved (AFDESIGN file also included).
I know this is a known problem, just thought I’d give some more examples which may help shed some light onto what could be happening.
2020-06-12_09-49-44.mp4
repeated-expanded-stroke-problem.afdesign