I can confirm this fixed the display problem for me! Probably still worth troubleshooting for the average Joe, but it's great that this works around it. Fixed the issue in both 2.80 and 2.82 Alpha.

1. Yes.
2. It's happening in every file, on every Grease Pencil object. Is it still worth uploading a file?

Good question! It's been a few months since I used the Grease Pencil feature, and just updated to Catalina in the last few days, so it seems likely it's associated with the OS. I might roll back to see if that fixes the issue. Seems like a Card + Drivers + OSX compatibility kind of problem, but I haven't seen anyone else online complaining about it yet so it may be very specific.