Yes, I'll fill out a bug report. It does depend on the zoom level, if you are zoomed out, it nicely shows the traffic layer above the MapPolyline. However, once you zoom in, it shows the traffic layer underneath the MapPolyline instead (depending on where you are testing this). I found this because my app is a navigation app for vehicles, so zooming out is not helpful since users need to see the roads up close. Thanks for getting back to me and confirming that this is a bug.

was meant to say MapPolyline is still rendered above the traffic layer*. Sorry

Yes, I'm familiar with that, but it's not applicable here. No errors are thrown. You can go back and retry hundreds of times and nothing changes, which tells me the issue isn't on my side but rather something failing in Apple's processing pipeline

Yes... I'm familiar with that, but it's not applicable here. No errors are thrown. You can go back and retry hundreds of times and nothing changes, which tells me the issue isn't on my side but rather something failing in Apple's processing pipeline... or it may take even longer to process?