Trying to test, I changed the url from a web http URL to a data URI. When I did that, it rendered correctly in 2D and 3D.
I don't believe it is a CORS problem because: 1) As stated the URL worked on 2D map, 2) I also tried a locally hosted image with a relative path, and 3) Looking at the new Symbol Playground I am seeing the same behavior. If you click on PictureMarkerSymbol you see a yellow square with black JS drawn over Southern California, but if you switch to 3D, it disappears.
Since it's not working on the esri playground I think it must be a code issue instead of a configuration issue. So is this a bug in the API the way the url property is used? Or do you have to include extra steps to make it render?