Yes, apparently it is an issue that still cannot be overcome, there have been several posts related to low quality in mobiles when clamped polygons are drawn.
On the other hand I have noticed that using 3d-tiles with classification have a similar behavior.
I have tested your sandcastle, finding the same fault in:
- Samsung S20 +
- Huawei P20 Lite
- Samsung Galaxy Tab
Some posts that could be related and that are closed:
It may be necessary to open another issue.
On the other hand I could notice that when painting on Terrain it has the same behavior. In the following link from sandcastle you can see that when zoom-in near the polygon “Draw Polygon” option also has this erratic behavior