Looks like the geometries of your data got simplified during the export. If you can share us with the data/webmap, we can investigate the problem on our end and provides a solution for it.
Sure, but it has happened with every layer I try. Does the map scale make a difference? I don't see why it would but I can't think of anything else.
If when creating the Mapboard I define a very small map scale, then the geometry looks much better. Is this the intended workflow? Since it's vector data, I'm not understanding why that would matter but it appears it does. Is it converted from raster at some point?
Thank you for preparing the data. We are able to reproduce the issue and will be looking for a solution for it. To answer your question, the vector data is not scale dependent and there is no raster to vector conversion during the process.
Vector data snaps to display pixel grid during export. So the issue is related to map export dpi setting. Try to bump up your setting to 300 dpi and see if the results meet your production requirements. You can also export the map in a large scale (increased level of detail) to have better results as you already know. If it is still an issue, we can increase the up limit in the incoming release.
I also have this issue and notice in AGOL - depending on the scale, the polygon will be simplified to a degree. When we pull in the data to illustrator it pulls the simplified polygon at the scale it will fit within the artboard.
It doesn't matter if you up the DPI because it is still pulling the simplified polygon. Only when you zoom way into a feature does it draw without simplification... though this doesn't allow you to download all of the features in the project area.