I was rendering in Vegas Pro 15-416 to the default Magix AVC/AAC Internet UHD render template AMD VCE option when I got these artifacts:
Adding a one frame crossfade produced these results, ceteris paribus.
With successful peer review, I would submit a bug report and go through all the analysis of O/S and video driver versions.
Big Note: This is not a decode problem. I replaced the source media six ways to Sunday.
I'm particularly interested to learn if the issue still exists in Vegas Pro 16.
Edit: 2019-10-31 Updated the project to include multiple points of failure.